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

[x7peeps]

Summary

Adds return_exceptions=True to asyncio.gather() in preprocess_context_references_async() to prevent a single failing reference from crashing all concurrent expansions.

Problem

When processing multiple context references (e.g. @url:, @file:) concurrently via asyncio.gather(), if any one reference raises an unexpected exception (e.g. cancellation, network error, or any uncaught exception from the reference chain), the entire gather fails and all other valid references are lost. This means a single bad URL reference in a user message can silently drop all other context refs.

Fix

• Add return_exceptions=True to the asyncio.gather() call at line 161
• Iterate results and handle Exception items by appending them as warnings (consistent with how the function already handles warnings)
• Continue processing remaining results normally
• Minimal diff: 7 insertions, 2 deletions

Testing

• Syntax check passed (py_compile)
• Behavior verified

[x7peeps]

Closing as superseded by #59954 (newer version covering context reference expansion + additional fixes in moa_loop and voice_mode).