ci: PR issue-linkage gate accepts only closing keywords, so multi-batch PRs cannot pass without wrongly closing the issue #12995

Surfaced on PR #12994 and, identically, on PR #12991 — both batches of #12979.

Problem

The Check PR links to its issue gate accepts only closing keywords:

##[error]PR body has no 'Resolves/Closes/Fixes #N' linkage.
##[error]Add 'Closes #12979' to the PR body.

It derives the expected issue number from the branch name (issue-12979-integrations-pool → 12979) and then demands a keyword that closes that issue on merge.

That leaves no way to express the common case of an issue delivered across several PRs. #12979 is the current example: 134 raw aiohttp.ClientSession( constructions, being converted in reviewable batches (#12982 pilot, #12991 utils, #12994 integrations), with ~98 still remaining in autobot-backend/ alone. Each batch legitimately advances the issue; none of them closes it.

The gate therefore forces a choice between two wrong outcomes:

1. Add Closes #12979 — which contradicts the closure rule that partial delivery never closes an issue, and would auto-close a live issue with most of its work outstanding.
2. Leave it off and ship with a red check — which is what both perf(http): route utils/ health and model probes through the shared pool (#12979) #12991 and perf(http): route CI/CD and VCS integrations through the shared pool (#12979) #12994 did.

Option 2 is the lesser evil, and the check is not in the required set for Dev_new_gui, so it does not block merge. But a permanently-red advisory check on every batch PR is noise that trains reviewers to ignore the gate, which defeats its purpose for the PRs where the linkage genuinely is missing.

Evidence

$ gh pr checks 12991 | grep "links to its issue"
Check PR links to its issue	fail	3s

$ gh pr checks 12994 | grep "links to its issue"
Check PR links to its issue	fail	2s

Both PR bodies reference #12979 repeatedly — in the title, in the thinking path, and in the verification section. The gate does not count any of it, because none of it uses a closing keyword.

Suggested fix

Accept a non-closing linkage keyword as satisfying the gate. Refs #N, Part of #N, or Advances #N all express "this PR belongs to issue N but does not complete it", and GitHub already renders them as a cross-reference without closing.

The gate's real intent — every PR must be traceable to an issue — is fully served by that. Requiring specifically a closing keyword conflates traceability with completion, and only the first of those is a property every PR should have.

Minimal change: extend the accepted pattern from (Resolves|Closes|Fixes) to also match (Refs|Part of|Advances), keeping the branch-derived issue number check unchanged so the linkage still has to point at the right issue.

Verification

Re-run the gate against PR #12994 with Refs #12979 in the body and confirm it passes, then against a PR body with no issue reference at all and confirm it still fails.