Playground preview: derive PR from the triggering run

[obenland]

What

The privileged pr-playground-preview.yml publisher currently reads the PR number and commit SHA out of the build artifact's name (ai-plugin-zip-pr<n>-<sha>). This change instead:

• Takes the head SHA from the workflow_run event (context.payload.workflow_run.head_sha).
• Resolves the pull request by matching that SHA against open PRs' head SHAs.
• Locates the build artifact by the exact name the trusted build derives from that PR and SHA, rather than by prefix.

The published preview and the comment target now follow from the commit that was actually built, not from artifact-supplied values.

Why

The build workflow runs on PR code with a read-only token; its artifacts (name and contents) are untrusted. Binding to the workflow_run head SHA keeps the publish step aligned with the triggering run.

Testing

• actionlint clean; embedded github-script syntax-checked.
• No producer change: the build already runs PR code by design, and the fix is entirely on the trusted consumer side.
• Worth a check on a fork-originated PR to confirm the PR resolves in the runner.

[Copilot]

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[jeffpaul]

Waiting to see that the generated playground env works before approving

[github-actions]

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Co-authored-by: obenland <obenland@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[github-actions]

✅ WordPress Plugin Check Report

✅ Status: Passed

📊 Report

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[obenland]

@jeffpaul Looks like GH actions are back online

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.57%. Comparing base (fc6bb2d) to head (3cecd11).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #978   +/-   ##
==========================================
  Coverage      74.57%   74.57%           
  Complexity      3132     3132           
==========================================
  Files            132      132           
  Lines          12213    12213           
==========================================
  Hits            9108     9108           
  Misses          3105     3105           

Flag	Coverage Δ
unit	74.57% <ø> (ø)

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