pr-states: fix fork-PR token + add run-ci label awareness

[hnyls2002]

Summary

• Switch pr-states.yml trigger from pull_request to pull_request_target so fork PRs receive a write-enabled GITHUB_TOKEN and the awareness comment update succeeds
• Show a red ❌ when the PR is missing the run-ci label (parallel to the existing run-ci-extra warning)
• Drop a redundant 4-line code comment in slash_command_handler.py (the deprecation message posted right below already states the rationale)

Background — fork PR token

• On pull_request events from a fork, GitHub forces GITHUB_TOKEN to be read-only — this is a platform-level security policy that ignores the permissions: pull-requests: write declaration in the workflow file
• pr-states / update-pr-body calls PATCH /repos/.../pulls/<n> which fails with 403 Resource not accessible by integration on every fork PR (reproduced on this PR's first run before the fix: Run 25913505322)
• 100% of fork PRs end up with a red pr-states / update-pr-body check in their checks list, which is confusing for external contributors

Fix — pull_request_target

• pull_request_target runs in the context of the base branch and grants the workflow a write-enabled token, so the API PATCH succeeds for both fork and non-fork PRs
• This workflow does not checkout PR head code — it only reads PR metadata via the GitHub API and PATCHes the PR body. The standard pull_request_target supply-chain concern (running untrusted fork code with a write token) does not apply

run-ci label awareness

• Without the run-ci label, pr-test.yml's gate (pr-gate.yml:57) fails and all downstream stages skip — but the awareness block previously linked to the (effectively empty) run, which was misleading
• Now mirrors the run-ci-extra pattern: show :x: **Missing run-ci label** instead of a stale link

Verification

• The pull_request_target switch cannot be observed on this PR's CI: GitHub runs the workflow file from the base branch, so until this is merged the fork PR still hits 403. The fix is reviewable as code; merge will activate it for the next fork PR

Follows up on #25387.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[hnyls2002]

/tag-and-rerun-ci