14 focused tests on the extracted helper
``_xai_oauth_exchange_code_for_tokens`` cover:
Core contract:
* ``code_verifier`` is on the wire (RFC 7636 §4.5).
* ``code_challenge`` + ``code_challenge_method=S256`` are echoed
(the #26990 defense-in-depth that makes xAI's token endpoint
stop rejecting valid exchanges).
* ``grant_type=authorization_code``, ``code``, ``redirect_uri``,
and ``client_id`` are all locked.
* Content-Type is ``application/x-www-form-urlencoded`` (xAI
rejects ``application/json`` on this endpoint).
* The supplied ``token_endpoint`` URL is used verbatim — no
hard-coded constant sneaks in via a future refactor.
* ``timeout_seconds`` is forwarded; floored at 20s.
Sanity guard:
* Empty ``code_verifier`` raises ``xai_pkce_verifier_missing``
with a link to #26990 — and NOTHING is sent. Leaking the auth
code to a server that can't redeem it is the wrong failure mode.
* Empty ``code_challenge`` omits only the defensive echo; the
standards-compliant ``code_verifier`` request still goes out so
RFC-compliant servers keep working.
Error surfacing:
* Non-200 responses include both ``HTTP <status>`` and the body
verbatim — disambiguates 400 (PKCE / bad request) from 403
(tier denied, see #26847).
* Transport errors are wrapped as ``AuthError`` with the
``xai_token_exchange_failed`` code, so the surrounding
``format_auth_error`` UI mapping still fires.
* Non-dict JSON payloads raise ``xai_token_exchange_invalid``.
* 200 happy path returns the parsed payload dict verbatim.
End-to-end wire-format guard:
* A real ``httpx.Client`` with a stub transport captures the bytes
on the wire and asserts every PKCE field round-trips through
``urlencode``. Catches a future refactor that swaps
``data=`` for ``json=`` (which xAI would silently reject).
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