Restore API error mapping for book placement failures

[leodvincci]

Restore API error mapping for book placement failures

Type: API behavior
Priority: P1

Summary

Old controller mapped errors to HTTP status; new flow throws unchecked exceptions with no mapping.

Impact

500s for expected errors (book not found, shelf full, bad input).

Evidence

BookController.java now delegates; no exception handling.

@PostMapping("/{bookId}/shelf")
public ResponseEntity<HttpStatus> placeBookOnShelf(
    @PathVariable Long bookId, @RequestBody BookShelfAssignmentRequest shelfAssignmentRequest) {
  bookFacade.placeBookOnShelf(bookId, shelfAssignmentRequest);
  return ResponseEntity.ok(HttpStatus.OK);
}

Proposed Fix

Add @ControllerAdvice or reintroduce explicit mapping.

Acceptance Criteria

• Errors map to 400/404/409 as appropriate.

Expected

Controller maps expected errors to correct HTTP status codes.

Actual

Unchecked exceptions bubble up as 500 for common error cases.