Differentiate exception types for 400/404/409

[leodvincci]

Differentiate exception types for 400/404/409

Type: API behavior
Priority: P1

Summary

All failures throw IllegalStateException, losing HTTP semantics.

Impact

Clients get 500s instead of correct 4xx.

Evidence

bookCommandUseCases.java lines 25, 29, 34.

if (shelfAssignmentRequest == null || shelfAssignmentRequest.shelfId() == null) {
  throw new IllegalStateException("Shelf id is required");
}

if (!shelfAccessPort.findShelfById(shelfAssignmentRequest.shelfId()).isPresent()) {
  throw new IllegalStateException(
      "Shelf not found with id: " + shelfAssignmentRequest.shelfId());
}

if (shelfAccessPort.isFull(shelfAssignmentRequest.shelfId())) {
  throw new IllegalStateException(
      "Shelf with id " + shelfAssignmentRequest.shelfId() + " is full");
}

Proposed Fix

Use IllegalArgumentException for bad input, NotFound for missing shelf, Conflict for full shelf; map with @ControllerAdvice.

Acceptance Criteria

• Invalid input → 400
• Missing shelf → 404
• Full shelf → 409

Expected

Errors map to 400 (bad input), 404 (not found), 409 (conflict).

Actual

All failures throw IllegalStateException and surface as 500 without mapping.