Fix layout-fragile git-root assertion in source code metadata test (#25855)

## Summary & Motivation

The `test_link_code_references_to_git_if_cloud_cloud_context[github]`,
`[gitlab]`, and
`test_link_code_references_to_git_if_cloud_override_cloud_context` cases
were failing in buildkite. The expected git URLs hardcoded
`dagster-oss/python_modules/dagster-cloud`, but
`link_code_references_to_git_if_cloud` derives the repository path as
the file path relative to the nearest `.git` ancestor (via
`_locate_git_root` + `AnchorBasedFilePathMapping`). That is
checkout-layout-dependent: it resolves to
`dagster-oss/python_modules/dagster-cloud/...` in an internal dev
checkout (so the test passed locally) but to
`python_modules/dagster-cloud/...` in buildkite, where the package sits
at the OSS-style layout under its own git root. The code is correct —
the test was layout-fragile.

The fix asserts only the layout-independent parts of the URL: the
branch-base prefix (`startswith`) and the stable
`dagster-cloud/.../file#Ln` tail (`endswith`). The repo prefix in
between (`dagster-oss/python_modules/` vs `python_modules/`) is exactly
the env-dependent segment that shouldn't be pinned — pinning it was the
original bug. It also drops the dead, wrong-depth `GIT_ROOT_PATH`
constant. Since this file is copybara-synced, the fix propagates to
`dagster-io/dagster`, whose identical copy has the same fragility.

## Test Plan

`tox -e py312-default -- -k link_code_references` — all 6 cases pass.
ruff and ty clean. The assertions no longer depend on where `.git` sits
relative to the package, so they hold in every layout (internal dev,
buildkite, OSS).

## Changelog

NOCHANGELOG

Internal-RevId: 75edf6efe4e5f540e1b9fe26a28ac3641b39cbc1

Author: sndagster

python_modules/dagster-cloud/dagster_cloud_tests/metadata_tests/test_source_code_metadata.py

@@ -20,7 +20,6 @@
 
 # path of the `dagster` package on the filesystem
 DAGSTER_CLOUD_PACKAGE_PATH = os.path.normpath(file_relative_path(__file__, "../../"))
-GIT_ROOT_PATH = os.path.normpath(os.path.join(DAGSTER_CLOUD_PACKAGE_PATH, "../../../"))
 
 # path of the current file relative to the `dagster` package root
 PATH_IN_PACKAGE = "/dagster_cloud_tests/metadata_tests/"
@@ -152,10 +151,10 @@ def test_link_code_references_to_git_if_cloud_cloud_context(
                         asset.metadata_by_key[key]["dagster/code_references"].code_references[-1],
                     )
 
-                    assert meta.url == (
-                        f"{source_control_branch_base}/dagster-oss/python_modules/dagster-cloud"
-                        + (expected_file_path[len(DAGSTER_CLOUD_PACKAGE_PATH) :])
-                        + f"#L{expected_line_number}"
+                    path_in_package = expected_file_path[len(DAGSTER_CLOUD_PACKAGE_PATH) :]
+                    assert meta.url.startswith(f"{source_control_branch_base}/")
+                    assert meta.url.endswith(
+                        f"dagster-cloud{path_in_package}#L{expected_line_number}"
                     )
 
 
@@ -225,10 +224,12 @@ def test_link_code_references_to_git_if_cloud_override_cloud_context() -> None:
                         asset.metadata_by_key[key]["dagster/code_references"].code_references[-1],
                     )
 
-                    assert meta.url == (
-                        "https://github.com/dagster-io/other-repo/tree/main/dagster-oss/python_modules/dagster-cloud"
-                        + (expected_file_path[len(DAGSTER_CLOUD_PACKAGE_PATH) :])
-                        + f"#L{expected_line_number}"
+                    path_in_package = expected_file_path[len(DAGSTER_CLOUD_PACKAGE_PATH) :]
+                    assert meta.url.startswith(
+                        "https://github.com/dagster-io/other-repo/tree/main/"
+                    )
+                    assert meta.url.endswith(
+                        f"dagster-cloud{path_in_package}#L{expected_line_number}"
                     )