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luoxu ed1f4de3a3 fix(ble_mesh): align GATTS read/write response handling with ATT

bt_mesh_bta_gatts_cb did not always answer ATT Read/Write Requests:
- READ: on a callback error it only logged a warning and sent nothing; a
  0-byte read (Read Blob at an offset equal to the value length) also sent
  nothing, although it is a successful empty read.
- WRITE: on a callback error it sent nothing, and a partial/zero write was
  treated as success.
- Both: when the handle was not found or the attribute had no read/write
  callback, the request was silently dropped.

An ATT Request must always be answered:

- READ: len >= 0 is success -> Read Response (a 0-byte read yields an empty
  value); len < 0 -> ATT Error Response carrying the callback's error code
  (-len, since BLE_MESH_GATT_ERR(x) == -x). The copy length is clamped to
  the source buffer size as a defensive bound. If the handle is unknown or
  the attribute has no read callback, respond with INVALID_HANDLE /
  READ_NOT_PERMITTED.
- WRITE: when need_rsp is set, always reply. len == write length -> Write
  Response; otherwise (negative ATT error, partial write, or 0) -> ATT
  Error Response (the negative code, or UNLIKELY for partial/0). If the
  handle is unknown or the attribute has no write callback, respond with
  INVALID_HANDLE / WRITE_NOT_PERMITTED. Write Without Response still sends
  no response.

A non-success status passed to BTA_GATTS_SendRsp is turned into an ATT
Error Response by the GATT layer (gatt_sr_process_app_rsp ->
gatt_send_error_rsp).

2026-08-03 11:13:28 +08:00