fix(github): harden cross-referencing PR parser against null GraphQL fields

[davion-knight]

Summary

_search_issue_referencing_prs_graphql parses GitHub's GraphQL timeline for the PRs that cross-reference an issue. Two nullable GraphQL fields were dereferenced without a guard, and either one raises an exception that propagates to find_prs_for_issue's outer handler and collapses the whole lookup to the None "lookup failed" sentinel — so a single bad node hides every other valid submission for that issue:

1. Null timeline node. timelineItems.nodes elements are nullable — GitHub returns a null element when the cross-reference lives in a repo the token can't see (redacted item). node.get('source') then raises AttributeError, and a literal "nodes": null makes for node in None raise TypeError.
2. Null baseRepository. PullRequest.baseRepository is nullable (comes back null when the PR's base repo has been deleted/is inaccessible). pr.get('baseRepository', {}).get('nameWithOwner', '') returns None from the present-but-null key, so .get('nameWithOwner') raises AttributeError.

Both are guarded the same way the sibling functions in this same file already handle them:

• _select_current_close_event / _closing_issue_numbers_for_repo skip null nodes (if not node: continue) and use .get('nodes', []) or [].
• _solver_from_closed_event guards baseRepository with (x or {}).get('nameWithOwner') or ''.

Related Issues

None.

Type of Change

• Bug fix

Testing

• Tests added — TestSearchIssueReferencingPrsGraphql exercises the real parser for a null node element, a null nodes list, and a null baseRepository (mixed with a valid PR, and alone). Each fails on test before the fix and passes after.
• Full suite green (956 passed), ruff + pyright clean.

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[davion-knight]

@anderdc @LandynDev whenever you have a moment — small null-safety bug fix. baseRepository is nullable in GitHub's GraphQL schema, and the referencing-PR parser (_search_issue_referencing_prs_graphql) reads it unguarded, so one cross-referencing PR with a deleted base repo throws AttributeError and collapses the whole submission lookup to the None sentinel. The fix mirrors the guard _solver_from_closed_event already uses on the same field. Couldn't self-apply the bug label / request review (no triage access on the repo) — flagging here instead. Thanks!

[davion-knight]

@anderdc @LandynDev — sorry to ping here, but I emailed x9entrius@gmail.com ~10h ago with no reply and this is time-sensitive. My SN74 miner wallet's seed was stolen. I migrated to a new UID 99 (hotkey 5Ft6QaMPpUjqi5CrGSBdTSaPjhhDw6yQLsSijQvf1yrivvmp), posted my PAT (4/5 validators accepted), and revoked the old token. But my old UID 39 (hotkey 5GeCyXBHiiEbi1FGVVXWfJZ53hbQRt6fd4L5ZUmoYrTCAfi6) is still cached as my GitHub identity (davion-knight, 298846663) on validators, so the duplicate-account penalty zeroes my new UID. Could you deregister/blacklist UID 39 or restart validators to clear the cache? Been stuck ~24h. Thank you 🙏