Fix patch coverage diff base for branches behind main

[glen-84]

Summary

• The patch-coverage job computed changed lines as git diff <pull_request.base.sha>...HEAD. That base SHA is captured when the webhook fires and goes stale once main advances, so for a PR that's behind main the diff swept in every commit merged to main in between, massively inflating the reported file list.
• It now diffs the merge commit's own parents (HEAD^1...HEAD^2), which yields the PR's true changes against the real fork point and matches GitHub's "Files changed" regardless of main drift.

Test plan

• On a real PR that was behind main, the old base produced 202 changed .cs files; HEAD^1...HEAD^2 produces 13, matching GitHub's Files-changed count.

[Copilot]

Pull request overview

Updates the patch-coverage workflow to compute diffs based on the PR merge commit’s parents, ensuring the reported changed lines/files match GitHub’s “Files changed” even when the PR branch is behind main.

Changes:

• Switch patch-coverage diff base from ${{ github.event.pull_request.base.sha }}...HEAD to HEAD^1...HEAD^2.
• Add inline workflow comments describing why the merge-parents diff is used.

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