Fix generated-output comparisons and fork comments

[schani]

Problems

1. The snapshot job compared the current base tip against the contributor branch tip. When master advanced after a branch forked, unrelated base changes appeared as modified/deleted generated files. PR Fix generated-output comparisons and fork comments #3077 incorrectly reported 67 differences for this reason.
2. workflow_run.pull_requests is empty for fork pull requests, so the trusted publisher rejected C#: add --use-records option for C# 9+ record types #3071's comparison artifact and never commented.

Fix

• Compare the exact base snapshot against GitHub's tested PR merge commit (github.sha). This mirrors GitHub's PR diff semantics and isolates changes introduced by the PR.
• Resolve the artifact's PR through the GitHub API and validate its base repository/branch and head repository/branch/SHA against trusted workflow_run data.

Verification

• The first Fix generated-output comparisons and fork comments #3077 run reproduced the bug: 27 modified and 40 deleted files.
• After switching to the tested merge commit, the same PR produced 0 changed files and updated its sticky comment to No generated-output differences.
• Fork validation was exercised against C#: add --use-records option for C# 9+ record types #3071 metadata and same-repository validation against fix(schema): merge array item types across allOf members #2997 metadata.
• Lint, workflow YAML parsing, and focused output-diff unit tests pass.

[github-actions]

No generated-output differences

✅ Generated outputs are unchanged between the PR base and head revisions.