fix(assistant): negotiate structured errors before SSE starts

[vitorhugo-dotnet]

Summary

Fixes the backend side of #96.

Root cause

POST /api/v1/assistant/chat produces text/event-stream, but validation and malformed-body errors happen before the controller returns an SseEmitter. The global exception handlers did not declare JSON/problem media types, so the original 400 could cascade into HttpMediaTypeNotAcceptableException when the client negotiated SSE.

Changes

• keep @NotNull conversationId and @NotBlank message
• explicitly negotiate application/problem+json and application/json for:
  • MethodArgumentNotValidException
  • HttpMessageNotReadableException
• keep provider/runtime errors inside the established SSE event flow
• add regression coverage for:
  • missing conversation ID
  • malformed UUID
  • blank message
  • valid request still negotiating SSE

Related frontend fix: companion PR in React-JobApplyTracker.

Fixes #96

[github-actions]

Qodana for JVM

3 new problems were found

Inspection name	Severity	Problems
Nullability and data flow problems	🔶 Warning	1
Mismatched query and update of 'StringBuilder'	🔶 Warning	1
Simplifiable conditional expression	🔶 Warning	1

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