feat: migrate chat backend to Semantic Kernel with tool calling support

Replace manual HTTP proxy in ChatController with Semantic Kernel's
OpenAI chat completion service pointed at CLIProxyAPI. Add extraction
plugin with validation function for structured field extraction from
natural language, enabling an agentic loop with auto-retry and
human-in-the-loop escalation.

- Add Microsoft.SemanticKernel 1.74.0 with OpenAI connector
- Create ExtractedFields schema and ValidationResult models
- Create ExtractionPlugin with [KernelFunction] validation
- Rewrite ChatController to use IChatCompletionService streaming
- Configure FunctionChoiceBehavior.Auto() for tool calling
- Preserve existing SSE contract (client unchanged)
- Update tests to mock SK services, add plugin and integration tests
- Archive multi-turn-conversations and migrate-to-semantic-kernel changes
- Sync specs for agent-extraction, semantic-kernel-integration, chat-streaming

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/ChatAgent.Api.Tests/ChatAgent.Api.Tests.csproj

@@ -11,6 +11,7 @@
     <PackageReference Include="coverlet.collector" Version="6.0.2" />
     <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.14" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="Microsoft.SemanticKernel" Version="1.74.0" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.2" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />