feat(chat): Create chat definition contract and support multiple outputs

[skarllot]

Pull Request

The issue or feature being addressed

• Need to support a definition of chat script and message parsing

Details on the issue fix or feature implementation

• Create IChatDefinition contract for configuring chat execution
• Move message definition to Chats context
• Rename AllowedModel to LlmModelType and move to Chats context
• Support multiple outputs

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

Attention: Patch coverage is 81.65680% with 31 lines in your changes missing coverage. Please review.

Project coverage is 39.37%. Comparing base (879d6a2) to head (310bb02).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...t/Operations/ReviewChanges/ReviewChatDefinition.cs	89.47%	8 Missing ⚠️
...ions/ProxyCompleteChat/ProxyCompleteChatHandler.cs	0.00%	8 Missing ⚠️
...ow/Operations/ProxyCompleteChat/AnthropicMapper.cs	0.00%	7 Missing ⚠️
.../Flow/Operations/ProxyCompleteChat/OpenAiMapper.cs	0.00%	7 Missing ⚠️
src/FlowPair/Chats/Infrastructure/IChatModule.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   39.19%   39.37%   +0.17%     
==========================================
  Files          56       58       +2     
  Lines        1064     1082      +18     
  Branches       91       91              
==========================================
+ Hits          417      426       +9     
- Misses        631      643      +12     
+ Partials       16       13       -3     

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