Suggested test: copilot/resolve-pull-request-review-thread-stale-noop

## Motivation

github/gh-aw#49648 — `resolve_pull_request_review_thread.cjs` now treats stale/invalid thread IDs and already-resolved threads as successful no-ops (`{ success: true, skipped: true }`) instead of raising an error, so `resolve-pull-request-review-thread` no longer fails a job when the target thread was resolved/deleted between agent decision and safe-output execution. The existing `test-copilot-resolve-pull-request-review-thread.md` only exercises the happy path (a valid, unresolved thread) driven by `workflow_dispatch` inputs.

## Proposed test

- **Workflow file**: `test-copilot-resolve-pull-request-review-thread-stale.md`
- **Trigger**: `workflow_dispatch` with `pull_request_number` and `thread_id` inputs
- **Engine**: copilot
- **Safe output**: `resolve-pull-request-review-thread`
- **Variant**: standard

## Minimal test prompt sketch

Pass a `thread_id` for a thread that is already resolved (or an intentionally invalid/stale GraphQL node ID) and have the agent attempt to resolve it. Assert the job completes successfully (no failure) rather than erroring, confirming the no-op/skip path.

## New fixtures or secrets needed

Needs a pre-resolved review thread ID to pass as input, or reuse an intentionally garbage ID to hit the "stale/invalid" null-node path. Same `GH_AW_TEST_PAT` secret as the existing resolve-thread test; no new fixture repo required.

## Notes

Complements `test-copilot-resolve-pull-request-review-thread.md` (happy path) by covering the regression fixed in #49648. Not a duplicate of any open suggestion.

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Compare the proposed test-copilot-resolve-pull-request-review-thread-stale.md workflow with test-copilot-resolve-pull-request-review-thread.md, including its workflow_dispatch inputs and GH_AW_TEST_PAT use. Add the stale or invalid thread scenario and verify that the resolve-pull-request-review-thread safe output completes successfully with no job failure.

Written by the indexing model from the issue text.