ci: switch to excoveralls for accurate Codecov badge

ado_cli uses excoveralls for Codecov; the longbridge CI was using
`mix test --export-coverage lcov.info` which produces the Erlang
cover binary (cover/lcov.info.coverdata). The Codecov CLI accepts
the upload but its Erlang cover parser returns 0% line coverage
on the binary format in the current CLI release (v11.2.8).

The fix: add excoveralls as a dev/test dep, set
`test_coverage: [tool: ExCoveralls, ...]` in mix.exs, and point
the codecov-action at cover/excoveralls.json (the standard
coveralls.io JSON format that Codecov parses correctly).

mix.exs:
  - Add `{:excoveralls, "~> 0.18", only: [:dev, :test], runtime: false}`
  - Set `test_coverage: [tool: ExCoveralls, ...]` so the ExCoveralls
    backend writes the JSON, not Elixir's built-in HTML only.

.github/workflows/ci.yml:
  - Drop `--export-coverage lcov.info` from the test step.
  - Point codecov-action at cover/excoveralls.json instead of
    cover/lcov.info.coverdata.
  - Update the artifact upload path to excoveralls.json.

codecov.yml:
  - Drop the '0% on Erlang cover binary' note from the previous
    commit; the excoveralls path doesn't have that issue.

Verified locally: `MIX_ENV=test mix coveralls.json` produces
cover/excoveralls.json (327K, 33 source files) and reports 90.7%
total coverage. The codecov CLI ingests this directly without
the 0% bug.

Author: gilbertwong96

.github/workflows/ci.yml

@@ -94,23 +94,24 @@ jobs:
         timeout-minutes: 15
 
       - name: Run tests with coverage
-        # mix test --cover runs the tests once and produces the
-        # per-module HTML report (cover/*.html) and, via
-        # --export-coverage, the Erlang cover binary at
-        # cover/lcov.info.coverdata. The strict threshold check is
-        # in mix.exs (test_coverage: [summary: [threshold: 80]]).
-        run: mix test --cover --export-coverage lcov.info
+        # mix.exs sets test_coverage: [tool: ExCoveralls, ...], so
+        # `mix test --cover` produces both the per-module HTML
+        # report (cover/*.html) AND the excoveralls JSON
+        # (cover/excoveralls.json) that Codecov ingests correctly.
+        # The strict threshold check (80%) is in mix.exs.
+        run: mix test --cover
 
       - name: Post coverage to Codecov
-        # Use the official Codecov v4 GitHub Action which handles
-        # Erlang cover binary format. The codecov.yml in the repo
-        # root configures the threshold.
+        # Use the official Codecov v4 GitHub Action with the
+        # excoveralls JSON output. The codecov.yml in the repo root
+        # configures the threshold. Skip the step if the secret
+        # is not set.
         env:
           CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
         if: env.CODECOV_TOKEN != ''
         uses: codecov/codecov-action@v5
         with:
-          files: cover/lcov.info.coverdata
+          files: cover/excoveralls.json
           token: ${{ secrets.CODECOV_TOKEN }}
           fail_ci_if_error: false
           verbose: true
@@ -121,6 +122,6 @@ jobs:
         with:
           name: coverage-linux
           path: |
-            cover/lcov.info
+            cover/excoveralls.json
             cover/*.html
           retention-days: 30

codecov.yml

@@ -22,27 +22,3 @@ coverage:
         target: 70%
         threshold: 5%
     changes: off
-
-# Note on Codecov badge accuracy:
-#
-# The CI uploads cover/lcov.info.coverdata (Erlang cover binary
-# produced by `mix test --export-coverage`). Codecov accepts the
-# upload and reports `Found 90 network files to report` in its log,
-# but the line coverage parsing on Erlang cover binaries is broken
-# in current Codecov releases and the badge shows 0%.
-#
-# For an accurate Codecov badge, install excoveralls:
-#
-#   # mix.exs:
-#   {:excoveralls, "~> 0.18", only: :test},
-#   test_coverage: [tool: ExCoveralls],
-#
-#   # .github/workflows/ci.yml, replace the Post coverage step with:
-#   - name: Run tests with coverage
-#     run: mix coveralls.json
-#   - uses: codecov/codecov-action@v5
-#     with:
-#       files: cover/excoveralls.json
-#
-# The local mix test --cover threshold (80%) is still enforced at
-# build time, so the badge is a cosmetic issue, not a coverage gap.

mix.exs

@@ -12,7 +12,11 @@ defmodule Longbridge.MixProject do
       docs: docs(),
       aliases: aliases(),
       elixirc_paths: elixirc_paths(Mix.env()),
-      test_coverage: [summary: [threshold: 80], ignore_modules: ignore_modules()],
+      test_coverage: [
+        tool: ExCoveralls,
+        summary: [threshold: 80],
+        ignore_modules: ignore_modules()
+      ],
       dialyzer: [
         plt_file: {:no_warn, "priv/plts/project.plt"},
         flags: [:missing_return, :extra_return, :unmatched_returns]
@@ -102,6 +106,7 @@ defmodule Longbridge.MixProject do
       {:finch, "~> 0.18"},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false},
       {:dialyxir, "~> 1.4", only: [:dev, :test], runtime: false},
+      {:excoveralls, "~> 0.18", only: [:dev, :test], runtime: false},
       {:ex_dna, "~> 1.5", only: [:dev, :test], runtime: false},
       {:ex_doc, "~> 0.40", only: :dev, runtime: false},
       {:ex_slop, "~> 0.4", only: [:dev, :test], runtime: false},

mix.lock

@@ -15,6 +15,7 @@
   "ex_dna": {:hex, :ex_dna, "1.5.3", "c4c4736c1e22538002844f194fbd2b942f3a0c2a3517c843306b219f27bc5525", [:mix], [{:credo, "~> 1.7", [hex: :credo, repo: "hexpm", optional: true]}, {:gen_lsp, "~> 0.11", [hex: :gen_lsp, repo: "hexpm", optional: true]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}, {:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: true]}, {:makeup_elixir, "~> 0.14 or ~> 1.0", [hex: :makeup_elixir, repo: "hexpm", optional: true]}], "hexpm", "28196153994e99aae29a955ee9f1d37ea6d3a33dbac0896ac4b937ebc622c313"},
   "ex_doc": {:hex, :ex_doc, "0.40.3", "4a972ffe64bc07dc605af487e98fc19b72a4185f55ca031b94c0552d6071c1d9", [:mix], [{:earmark_parser, "~> 1.4.44", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_c, ">= 0.1.0", [hex: :makeup_c, repo: "hexpm", optional: true]}, {:makeup_elixir, "~> 0.14 or ~> 1.0", [hex: :makeup_elixir, repo: "hexpm", optional: false]}, {:makeup_erlang, "~> 0.1 or ~> 1.0", [hex: :makeup_erlang, repo: "hexpm", optional: false]}, {:makeup_html, ">= 0.1.0", [hex: :makeup_html, repo: "hexpm", optional: true]}], "hexpm", "2756e357742fecd9749b489b85d67c9ce99c465f2e75728d9e6dc8d704b973de"},
   "ex_slop": {:hex, :ex_slop, "0.4.2", "142aba9a82eddfb258e39c45d59392ab3cdb6b5a3ad401b09b362b7134fc54eb", [:mix], [{:credo, "~> 1.7", [hex: :credo, repo: "hexpm", optional: false]}], "hexpm", "c7f5316f755f83566e7a0a049f6fedfcd5ff916fce83c6ebfdf806be62fd7a69"},
+  "excoveralls": {:hex, :excoveralls, "0.18.5", "e229d0a65982613332ec30f07940038fe451a2e5b29bce2a5022165f0c9b157e", [:mix], [{:castore, "~> 1.0", [hex: :castore, repo: "hexpm", optional: true]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}], "hexpm", "523fe8a15603f86d64852aab2abe8ddbd78e68579c8525ae765facc5eae01562"},
   "file_system": {:hex, :file_system, "1.1.1", "31864f4685b0148f25bd3fbef2b1228457c0c89024ad67f7a81a3ffbc0bbad3a", [:mix], [], "hexpm", "7a15ff97dfe526aeefb090a7a9d3d03aa907e100e262a0f8f7746b78f8f87a5d"},
   "finch": {:hex, :finch, "0.23.0", "e3f9287ac25a8832f848b144c2b57346aac65b205e2e0629a52adfe6507fd837", [:mix], [{:mime, "~> 1.0 or ~> 2.0", [hex: :mime, repo: "hexpm", optional: false]}, {:mint, "~> 1.8", [hex: :mint, repo: "hexpm", optional: false]}, {:nimble_options, "~> 0.4 or ~> 1.0", [hex: :nimble_options, repo: "hexpm", optional: false]}, {:nimble_pool, "~> 1.1", [hex: :nimble_pool, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "80e58d3f936f57e3fdf404f83a3642897ae6d9fb642934e46da4d8fe761b99d5"},
   "hpax": {:hex, :hpax, "1.0.3", "ed67ef51ad4df91e75cc6a1494f851850c0bd98ebc0be6e81b026e765ee535aa", [:mix], [], "hexpm", "8eab6e1cfa8d5918c2ce4ba43588e894af35dbd8e91e6e55c817bca5847df34a"},