upload-codescene-coverage: add behavioural test for skipped-step suppression

[coderabbitai]

Summary

Add behavioural coverage for skipped-step suppression in .github/actions/upload-codescene-coverage check mode.

test_skipped_gate_suppresses_all_following_steps currently reads if: expressions from action.yml. It verifies the manifest structure but does not execute a pull-request context where the changed-line coverage gate is skipped.

Required change

Execute a skipped check-mode scenario for a pull request whose base branch is not the default branch. Verify that each subsequent coverage-related step is not executed.

Rationale

GitHub Actions evaluates composite-action if: expressions at runtime. A manifest-only assertion cannot prove that the complete skip path suppresses downstream steps.

Affected areas

• .github/actions/upload-codescene-coverage/action.yml
• .github/actions/upload-codescene-coverage/tests/test_check_mode.py
• Test harnesses that execute the composite action, if required

Acceptance criteria

• The test executes a non-default pull-request base-branch context in mode: check.
• The test verifies the skip warning.
• The test proves that coverage-file selection, CLI installation, upload, cache, and check steps do not execute.
• The test remains isolated from the network.

References

• Pull request: Skip inapplicable CodeScene coverage checks and surface diagnostics (#395) #399
• Requesting comment: Skip inapplicable CodeScene coverage checks and surface diagnostics (#395) #399 (comment)
• Requested by: @leynos

[coderabbitai]

🔗 Related PRs

leynos/ddlint#287 - Wire the CodeScene coverage gate into CI [merged]
leynos/rstest-bdd#583 - Restore the coverage ratchet (#582) [merged]
leynos/pylint-pypy-shim#34 - Wire the CodeScene coverage upload into CI [merged]
leynos/episodic#182 - Wire CodeScene coverage upload via the shared actions [merged]
leynos/netsuke#417 - Enable the CodeScene mode: check coverage gate [merged]
#399 - Skip inapplicable CodeScene coverage checks and surface diagnostics (#395) [open]

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