fix(agent): add return_exceptions to asyncio.gather in context ref expansion

[x7peeps]

Summary

Add return_exceptions=True to asyncio.gather() in preprocess_context_references_async and guard the result loop against BaseException.

Problem

agent/context_references.py called asyncio.gather() without return_exceptions=True. If one reference expansion (e.g., a URL fetch or file read) fails, the exception propagates immediately and crashes the entire gather, aborting the agent turn — even when other references would have resolved fine.

Fix

1. Added return_exceptions=True to the gather call.
2. Updated the result iteration to check isinstance(result, BaseException) and append a warning instead of crashing.

Testing

• Syntax check passed (py_compile)
• Behavior verified

[alt-glitch]

This was generated by AI during triage.

Related: #59954 (the current open sibling with the same return_exceptions=True gather guard) and the wider preprocess_context_references_async gather-guard cluster (#57780 now closed-unmerged, #55207 the merged in-file concurrent-@-expansion). This is the clean single-file version of the shared hunk — verified the fix is not yet on main. Not a duplicate (subset/variant of a larger open PR is related, never dup); a human should pick the canonical entrant.

[x7peeps]

Closing as duplicate