Allow updates from the owner side of ManyToOne/ManyToMany relationships

[ruthst00]

Fixes #1810

The issue GH-1810 still existed in SDR 5.0.7 (and the current 5.2.0-1810-SNAPSHOT branch). The bug: when a client sends a PUT/POST/PATCH/DELETE to the inverse (mappedBy) side of a JPA bidirectional association (e.g. PUT /authors/{id}/books where Author.books has @ManyToMany(mappedBy="authors")), SDR would return HTTP 204 implying success — but JPA silently ignores writes on the non-owning side, so no database changes were made.

The fix (commit d7ffd2ce) adds two new files and modifies two existing ones:

1. InverseSideAssociationDetector (new) — uses ClassUtils.forName + reflection to detect a non-empty mappedBy attribute on @OneToMany, @ManyToMany, or @OneToOne annotations. Gracefully returns false when jakarta.persistence is absent from the classpath.

2. RepositoryPropertyReferenceController — doWithReferencedProperty now calls InverseSideAssociationDetector.isInverseSide() before any write operation. If the property is the inverse side, it throws HttpRequestMethodNotSupportedException which the existing @ExceptionHandler converts to HTTP 405 Method Not Allowed with Allow: GET and a descriptive message directing the caller to update the owning side instead. GET requests are unaffected.

3. InverseSideAssociationDetectorUnitTests (new) — 6 unit tests covering all three annotation types with/without mappedBy.

4. RepositoryPropertyReferenceControllerIntegrationTests — 4 new JPA integration tests verifying PUT/POST on the inverse side throw the exception, GET still works, and PUT on the owning side continues to work normally. All 15 tests (9 unit + 6 integration) pass.

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