ATT server incorrectly handles authorization during Write Long procedure

When operating in the server role, Nimble incorrectly handles authorization checks during a Write Long Procedure. Instead of rejecting an unauthorized ATT_PREPARE_WRITE_REQ, it accepts it and only returns an error during the Execute Write phase.

Current behavior:
1. Client sends an ATT_Prepare_Write_Request.
2. Nimble responds with ATT_Prepare_Write_Response packet
3. Client sends an ATT_Execute_Write_Request
4. Nimble responds with an ATT_Error_Response (Insufficient Authorization)

Expected behavior:
Based on Core Specification 6.3, Vol 3, Part G, Section 4.9.4 (Write Long Characteristic Value), the server shall send an ATT_ERROR_RSP in response to an ATT_PREPARE_WRITE_REQ if the client has insufficient authorization.

1. Client sends an ATT_Prepare_Write_request
2. Nimble responds with ATT_Error_Response (Insufficient Authorization)

This affects GATT/SR/GAW/BI-11-C qualification test case

Start by running or locating the GATT/SR/GAW/BI-11-C qualification test and trace the server's handling of ATT_Prepare_Write_Request authorization. Compare the prepare and execute phases described in the issue; done means an unauthorized prepare request receives ATT_Error_Response with Insufficient Authorization immediately.

Written by the indexing model from the issue text.