Stop leaking raw exception messages to API clients

[devops-thiago]

Background

GlobalExceptionHandler (src/main/java/br/com/arquivolivre/myjavagenie/controller/GlobalExceptionHandler.java) returns ex.getMessage() verbatim to clients for IllegalArgumentException, ModelTimeoutException, ModelInvocationException, ModelInitializationException, and the generic handler. These messages are built from provider/exception internals, e.g. "Language model generation failed: " + e.getMessage() in QueryService.generateWithTimeout, which can embed upstream API error details.

Impact

• Internal details (base URLs, request fragments, provider error payloads, filesystem paths) can leak to API consumers and the chat UI.
• Error handling is inconsistent: some paths sanitize via LogSanitizer, but the client-facing body does not.
• The chat UI surfaces these messages directly in the error banner, so a user can see provider internals.

How to reproduce

1. Point model.self-hosted.base-url at a closed port (or use an invalid API key).
2. Ask a question via POST /api/chat/query.
3. Observe the response body contains the raw connection/authentication error message from the provider instead of a generic message.

Where the fix should land

In GlobalExceptionHandler, map each exception type to a stable, user-facing message while keeping the current HTTP statuses:

• Timeout → "The request timed out while generating the answer. Please try again."
• Invocation/initialization → "The language model is temporarily unavailable. Please try again later."
• Generic → "An unexpected error occurred."

Keep full detail in the logs (the handlers already log with the exception object).

Files touched

• src/main/java/br/com/arquivolivre/myjavagenie/controller/GlobalExceptionHandler.java
• Tests under src/test/java/... (new GlobalExceptionHandlerTest or updates to existing controller tests)

Acceptance criteria

• Client error bodies contain no raw exception text or provider internals.
• Logs still contain the full stack trace and cause.
• HTTP status codes unchanged (400/504/503/500).
• Tests assert generic messages and preserved statuses; mvn test and mvn spotless:check pass.