[P2][errors] error.code is set to the whole human message for {status:"fail", data:{error:"<sentence>"}} responses #145

Status

Confirmed live on 2026-09-13 against production, with origin/main 4bd2900 code plus the #101 branch. The defect is in error_from_response, which is unchanged on main.

Defect

oilpriceapi/exceptions.py error_from_response:

code_value = _first_value(sources, "code", "error_code", "type")
if code_value is None and isinstance(nested_data_error, str):
    code_value = nested_data_error

When the API answers with the render_fail envelope, data.error is a human sentence, not a machine code. The SDK copies that sentence into error.code and error.machine_code. Anything that branches or aggregates on error.code sees a unique free-text string per request, for example one that embeds the requested slug.

Repro

import os
from oilpriceapi import OilPriceAPI
from oilpriceapi.exceptions import DataNotFoundError

with OilPriceAPI(api_key=os.environ["OILPRICEAPI_KEY"], max_retries=1) as client:
    try:
        client.request("GET", "/v1/fuel-surcharge/nope/latest")
    except DataNotFoundError as error:
        print(repr(error.code))

Output on 2026-09-13:

code: "Unknown carrier 'nope'. Covered carriers: odfl, saia, estes, xpo, abf, tforce, averitt, southeastern-freight."
message: "Unknown carrier 'nope'. Covered carriers: odfl, saia, estes, xpo, abf, tforce, averitt, southeastern-freight."

Wire body: {"status":"fail","data":{"error":"Unknown carrier 'nope'. ...","covered_carriers":[...],"hint":"..."}}.

Every V1::FuelSurchargeController 400/404 uses this shape, and so do other render_fail callers in the API.

Expected

When data.error is a sentence rather than a code-shaped token, leave error.code as None (or derive a stable code from the status). Keep the sentence in message. A narrow fix: only fall back to data.error when it matches something like ^[A-Z][A-Z0-9_]*$.

Before changing this, check tests/unit/test_error_contract.py for any pinned legacy fixture that relies on the fallback.