PF06: Spring Boot API Error Fix & Tests

PF06 is a self-initiated isolated debugging sample built with Java 21 and Spring Boot.

An input-validation path returned a generic 500 response, making client behavior and regression checks ambiguous.

I reproduced the same request, narrowed the failure to validation and exception mapping, applied a focused patch, and added response-contract tests.

• Input-validation 500 reproduced

• 400 invalid-input response

• 409 conflict and 201 create responses

• Five MockMvc tests

The sample demonstrates reproduction, root-cause isolation, a narrow patch, and regression protection. It is not evidence from a client production system and does not imply access to an inaccessible production environment.

Self-initiated isolated debugging sample. The bug, patch, responses, and tests were created for demonstration and are not paid or verified client work.