test: add artifacts-many-paths to validate ARG_MAX fix

- Add test case with 9000 files of 255-char names (~2.3 MB total
  path size) to verify rsync --files-from handles large artifact
  sets that would exceed ARG_MAX (~2 MB) as CLI arguments
- Assert "exported artifacts" in stdout to catch silent rsync
  failures hidden by || true
- Assert no FAIL in stderr to ensure consume-artifacts receives
  all 9000 files via artifact transfer
- Requires --shell-isolation so artifact transfer is exercised
  rather than the shared cwd being used directly

Author: Paul-Goulpie

tests/test-cases/artifacts-many-paths/.gitignore

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+/artifact_*
+/.gcl-many-paths/

tests/test-cases/artifacts-many-paths/.gitlab-ci.yml

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+---
+# Creates 9000 files whose names are 255 characters long.
+# With the old approach (paths passed as rsync arguments), the total argument
+# size (~2.3 MB) exceeded ARG_MAX (~2 MB) and rsync failed silently.
+# With the new approach (--files-from + bash for loop), there is no limit on
+# the number of files.
+produce-artifacts:
+  stage: build
+  script:
+    - |
+      python3 -c "
+      import os
+      padding = 'x' * 240
+      for i in range(9000):
+          name = f'artifact_{i:05d}_{padding}'
+          fd = os.open(name, os.O_CREAT | os.O_WRONLY, 0o644)
+          os.close(fd)
+      "
+  artifacts:
+    paths:
+      - artifact_*
+
+consume-artifacts:
+  stage: test
+  script:
+    - count=$(find . -maxdepth 1 -name 'artifact_*' | wc -l)
+    - if [ "$count" -ne 9000 ]; then echo "FAIL expected 9000 artifacts, got $count"; exit 1; fi

tests/test-cases/artifacts-many-paths/integration.test.ts

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+import {WriteStreamsMock} from "../../../src/write-streams.js";
+import {handler} from "../../../src/handler.js";
+import {initSpawnSpy} from "../../mocks/utils.mock.js";
+import {WhenStatics} from "../../mocks/when-statics.js";
+
+beforeAll(() => {
+    initSpawnSpy(WhenStatics.all);
+});
+
+// Validates that --files-from works correctly when the number of artifact files
+// would exceed ARG_MAX if their paths were passed directly as rsync arguments
+// (9000 files × ~257 bytes ≈ 2.3 MB > ARG_MAX ~2 MB).
+test.concurrent("artifacts-many-paths --shell-isolation 9000 artifacts via --files-from", async () => {
+    const writeStreams = new WriteStreamsMock();
+    await handler({
+        cwd: "tests/test-cases/artifacts-many-paths",
+        shellIsolation: true,
+        stateDir: ".gcl-many-paths",
+    }, writeStreams);
+
+    // Ensures artifacts were actually exported (not silently swallowed by || true).
+    expect(writeStreams.stdoutLines.join("\n")).toMatch(/produce-artifacts.*exported artifacts/);
+    // Ensures consume-artifacts found all 9000 expected files.
+    expect(writeStreams.stderrLines.join("\n")).not.toMatch(/FAIL/);
+}, 120_000);