Mark PR WPT results as pending while a new run is computing

[nicoburns]

Summary

After pushing to a PR there's a window where the WPT results in the description are stale with no indication of it. This adds a pending notice at the start of each WPT run, keeping the old results visible.

The "Post WPT results" workflow now also triggers on workflow_run: in_progress, with a new mark-pending job that runs wpt_diff_to_pr.py --pending. (The pending update lives in this trusted workflow_run workflow, not the WPT job itself, because the WPT job runs with the read-only fork-PR token.)

--pending inserts a > [!NOTE] alert under the ## WPT results heading, delimited by new <!-- wpt-pending-start/end --> markers nested inside the existing results markers:

<!-- wpt-results-start -->
## WPT results

No changes in test results compared to `main`.

<sub>Generated by the [WPT workflow](https://github.com/DioxusLabs/blitz/actions/runs/31736695796).</sub>
<!-- wpt-results-end -->

Because the notice is nested inside the results section, posting the final results (which replaces everything between the results markers) removes it automatically with no extra logic. If there are no previous results yet, --pending creates the results section with a "results will be posted here" notice instead.

A mark-failed job (on completed with conclusion == 'failure') runs --failed, which replaces the pending notice with a > [!WARNING] failure notice so a failed run doesn't leave a permanent "being computed" message. Cancelled runs are deliberately not handled: cancellation normally comes from cancel-in-progress when a new run starts, and clearing the notice there would race with the new run's mark-pending.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/549f86c7156a4b59aa01c59657fd74b2
Requested by: @nicoburns

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring