test: warn on fix PRs without e2e regression coverage (#9880)

## Summary

Add a CodeRabbit pre-merge warning for fix-like PRs that do not update
`browser_tests/` and do not explain why no end-to-end regression test
was added.

Requested by Christian

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9880-test-warn-on-fix-PRs-without-e2e-regression-coverage-3226d73d3650816eb3e1c1c7d0824edd)
by [Unito](https://www.unito.io)

.coderabbit.yaml

@@ -12,3 +12,14 @@ reviews:
       - comfy-pr-bot
       - github-actions
       - github-actions[bot]
+  pre_merge_checks:
+    custom_checks:
+      - name: End-to-end regression coverage for fixes
+        mode: warning
+        instructions: |
+          Pass if at least one of the following is true:
+          1. Neither the PR title nor any commit subject in the PR uses bug-fix language such as `fix`, `fixed`, `fixes`, `fixing`, `bugfix`, or `hotfix`.
+          2. The PR changes at least one file under `browser_tests/`.
+          3. The PR description includes a concrete, non-placeholder explanation of why an end-to-end regression test was not added.
+
+          Fail otherwise. When failing, mention which bug-fix signal you found and ask the author to either add or update a Playwright regression test under `browser_tests/` or add a concrete explanation in the PR description of why an end-to-end regression test is not practical.