fix: harden reviewer-bot state locking and triage escalation

[PLeVasseur]

Summary

• Fixes the burst assignment race by introducing a durable in-repo lease lock on state issue 📊 Reviewer Bot State (DO NOT CLOSE) #314 with optimistic concurrency (ETag plus If-Match).
• Refactors state issue persistence to preserve explicit YAML and lock marker blocks so lock metadata cannot be clobbered by save_state().
• Keeps read-level reviewers assignable as designated reviewers, adds truthful reviewer-request failure messaging for 422, and adds mandatory triage escalation handling.

Root Cause and Lock Strategy

• Reviewer-bot runs were serialized per PR/issue key, but all runs still wrote to the same state issue body.
• Parallel pull_request_target:labeled runs could overwrite each other in a last-writer-wins pattern.
• This change adds lease acquire/release with retry/backoff, stale lease takeover, and hard failure on lock timeout or state persistence failure.

State Contract and Preserve Update Strategy

• Adds explicit parse/render markers for state and lock blocks:
  • <!-- REVIEWER_BOT_STATE_START --> ... <!-- REVIEWER_BOT_STATE_END -->
  • <!-- REVIEWER_BOT_LOCK_START --> ... <!-- REVIEWER_BOT_LOCK_END -->
• Adds parse_state_yaml_from_issue_body, parse_lock_metadata_from_issue_body, and render_state_issue_body helpers.
• save_state() now re-reads latest body and uses conditional patching (If-Match) instead of blind body rewrites.

Mandatory Triage Approval Workflow

• Adds idempotent label ensure-create behavior for triage approver required.
• When a designated reviewer with read-level access approves:
  • marks review complete immediately
  • applies escalation label
  • posts one escalation ping comment per review cycle
• First triage+ APPROVED satisfies mandatory approval, records satisfier metadata, and removes the escalation label.

Validation

• uv run ruff check --fix
• uv run pytest .github/reviewer-bot-tests (89 passed)

Runbook Metadata

• Incident PRs: #246 [Coding Guideline]: Do not access memory using a pointer with an incorrect provenance #264, [Coding Rule]: assure visibility of unsafe keyword in unsafe code #358, [Coding Guideline]: Do Not Depend on Function Pointer Identity #256, [Coding Guideline] Revise guideline on unsafe code in macros #368
• State issue: 📊 Reviewer Bot State (DO NOT CLOSE) #314

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[felix91gr]

@plaindocs would you be able to review this? I don't know much about GH Actions' concurrency primitives

[plaindocs]

@felix91gr I'm in the same boat. But if we need a scapegoat to click "approve" and hope I'm game :-D

[felix91gr]

I don't know much about GH Actions' concurrency primitives

Now I do - they suck 🤣 that's why Pete has to implement a mutex and a whole data structure just to make GHA behave.