`post_comment` can target the wrong PR when a head branch has multiple PRs

[KSmanis]

This issue was identified and reported by gpt-5.6-sol. I independently
validated it using the reproduction below.

Bug

In v4.1 (5d8df5979747514c914e1c5a12335a7cf9a2745f), post_comment
resolves GITHUB_PR_RUN_ID to an owner/head branch, then selects a matching PR
instead of using the PR associated with the workflow run.

When one head branch has PRs against multiple bases, the artifact can be posted
to the wrong PR.

Reproduction

https://github.com/KSmanis/python-coverage-comment-action-wrong-pr-repro

PR #1's producer run
automatically triggered this
workflow_run,
which resolved PR #2. The resulting comment on PR #2 says
Generated for PR #1.

Expected

Post to the PR associated with the workflow run, or fail when ambiguous.

Suggested fix

Use the workflow run's associated PR, with branch lookup only as a fallback.