Add System.Text.Json source generators for AOT-compatible serialization

[GmausDev]

Summary

This PR implements P0 priority performance improvements by adding System.Text.Json source generators, eliminating runtime reflection for all JSON serialization/deserialization operations.

Changes

• New file: Serialization/CompactifAIJsonContext.cs - Source-generated JSON context with all 16 DTO types registered
• CompactifAIClient.cs - Updated to use CompactifAIJsonContext.Default instead of runtime JsonSerializerOptions
• ChatModels.cs - Changed ToolFunction.Parameters from object? to JsonElement? for source generator compatibility
• CompactifAI.Client.csproj - Added IsTrimmable and IsAotCompatible flags
• ServiceCollectionExtensions.cs - Added AOT warning attributes to configuration-binding method
• ModelsTests.cs - Updated all tests to use source-generated context

Benefits

Improvement	Expected Gain
Zero reflection serialization	2-5ms latency reduction
Cached type metadata	Reduced GC pressure
Compile-time validation	Catch property name mismatches at build time
AOT compatibility	Ready for ahead-of-time compilation
IL trimming support	Smaller deployment sizes

Test Plan

• All 38 existing unit tests pass
• Build succeeds with 0 warnings
• Source generators run at compile time (no runtime reflection)

Breaking Changes

• ToolFunction.Parameters changed from object? to JsonElement? - More explicit typing, use JsonSerializer.SerializeToElement() to create values