fix(agent): strip finish_reason and internal fields before API calls

[Bartok9]

Summary

Fixes #134

Mistral API returns 422 error on the second message because internal fields like finish_reason are being sent in the request.

Error

Error code: 422 - {'detail': [{'type': 'extra_forbidden', 
'loc': ['body', 'messages', 2, 'assistant', 'finish_reason'], 
'msg': 'Extra inputs are not permitted', 'input': 'stop'}]}

Root Cause

finish_reason is response metadata from the API, not a valid request field. The code was storing it in messages for trajectory/debugging purposes, but then copying those messages (including the field) when making subsequent API calls.

Fix

Strip internal-only fields (finish_reason, reasoning) from message copies before sending to the API. Applied to all three places where API messages are constructed:

1. flush_memories() API call
2. Summary generation API call
3. Main conversation API call

Fields Affected

• finish_reason - Now stripped (caused the Mistral 422)
• reasoning - Was already being stripped (for different reasons)

Testing

• Verified the fix applies to all API call sites
• Internal fields are still preserved in local messages for debugging/trajectory storage

[teknium1]

Thanks for the report, @Bartok9! This issue was already largely fixed by PR #253 (commit 7a0b377 by @Sertug17), which stripped finish_reason from API messages in both flush_memories() and run_conversation().

Your PR did catch one remaining gap though — _handle_max_iterations() was still doing a bare messages.copy() without stripping internal fields. We've patched that in commit 41adca4, which now strips finish_reason and reasoning consistently across all three API call sites.

Closing this since the fix is now fully covered, but appreciate you spotting the missing code path!