Function to_handler_error 

pub fn to_handler_error(e: Error) -> (StatusCode, Json<ErrorResponse>)

Convert a [BackendError] (from the service layer) into the best-fitting (StatusCode, Json<ErrorResponse>) pair returned by every handler. The canonical call shape is .map_err(to_handler_error)? at the end of each service call.

Status mapping (most-specific first):

• NotFound, SessionNotFound, ConfigurationNotFound → 404
• Conflict, ConfigurationAlreadyExistsError → 409
• BadRequest, InvalidPattern, UnsupportedBackend, InvalidNodeId → 400
• AuthenticationTokenNotFound, JwtMalformed → 401
• InsufficientResources → 422
• CsmError { status, .. } → that backend status (verbatim) when it’s a valid HTTP code, else 502 Bad Gateway
• NetError(rqe) where rqe.is_timeout() → 504 Gateway Timeout (manta-server → CSM hop)
• everything else → 500 Internal Server Error

pub (rather than pub(crate)) so the integration tests in crates/manta-server/tests/ can exercise the mapping directly.