Add Events (Like Tool Usage, Thinking, etc.) to Agent API and Return Message Types

[JSv4]

Initial stab at exposing thoughts, tool use, and content back to frontend for more modern chat experience.

[codecov]

Codecov Report

Attention: Patch coverage is 85.40773% with 34 lines in your changes missing coverage. Please review.

Project coverage is 90.02%. Comparing base (33dfbf0) to head (2f77f0e).
Report is 95 commits behind head on main.

Files with missing lines	Patch %	Lines
opencontractserver/annotations/models.py	77.52%	20 Missing ⚠️
opencontractserver/corpuses/models.py	90.00%	7 Missing ⚠️
opencontractserver/llms/agents/agent_factory.py	82.92%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #392      +/-   ##
==========================================
- Coverage   91.28%   90.02%   -1.26%     
==========================================
  Files         309      323      +14     
  Lines       15805    17451    +1646     
==========================================
+ Hits        14427    15711    +1284     
- Misses       1378     1740     +362     

Files with missing lines	Coverage Δ
opencontractserver/annotations/json_types.py	100.00% <100.00%> (ø)
...server/annotations/migrations/0033_noterevision.py	100.00% <100.00%> (ø)
...conversations/migrations/0003_chatmessage_state.py	100.00% <100.00%> (ø)
opencontractserver/conversations/models.py	93.22% <100.00%> (+1.06%)	⬆️
...corpus_md_description_corpusdescriptionrevision.py	100.00% <100.00%> (ø)
opencontractserver/llms/agents/core_agents.py	83.04% <ø> (-4.16%)	⬇️
...encontractserver/llms/agents/llama_index_agents.py	92.05% <ø> (+3.39%)	⬆️
...encontractserver/llms/agents/pydantic_ai_agents.py	70.63% <ø> (-14.93%)	⬇️
opencontractserver/llms/agents/timeline_schema.py	100.00% <ø> (ø)
...ontractserver/llms/agents/timeline_stream_mixin.py	43.47% <ø> (ø)
... and 28 more

... and 6 files with indirect coverage changes

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