[ISSUE-3779][test] Deepen GlobalExceptionHandlerTest with 501/500/503/type-mismatch/unreadable-body branches

[yyqdbngt]

Motivation

The existing GlobalExceptionHandlerTest covers business, LLM-gateway, 404/405/406 mappings, but five handler branches were still unverified through the advice: unsupported operations, the generic catch-all, Prometheus failures, request-parameter type mismatches and unreadable request bodies.

Changes

• unsupportedOperationReturns501Envelope: UnsupportedOperationException maps to HTTP 501 with the exception message preserved.
• unexpectedExceptionReturns500Envelope: the catch-all maps arbitrary runtime failures to HTTP 500 with the neutral Internal Server Error message (no leak).
• prometheusExceptionPreservesStatusAndMessage: a Prometheus failure keeps its status code and message.
• typeMismatchParameterReturns400InvalidParameter: a non-numeric path variable for an int parameter is reported as 400 with the generic parameter message.
• unreadableBodyReturns400InvalidBody: a malformed JSON body is reported as 400 with the Invalid request body message.

Verification

mvn -B test -Dtest=GlobalExceptionHandlerTest
[INFO] Tests run: 11, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS