refactor: drop the unreachable XOAUTH2 .authenticate fallback

T3-21: aioimaplib's IMAP4 always has .xoauth2 and never .authenticate, so the
fallback branch and its _sasl_xoauth2 helper could not run. call
mail.xoauth2 directly.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/aiomail/auth.py

@@ -1,5 +1,4 @@
 """authentication mechanisms for the IMAP client: `PasswordAuth` (LOGIN), `OAuth2Auth` (XOAUTH2)."""
-import base64
 import logging
 from typing import Awaitable, Callable, Optional, Protocol, Union, runtime_checkable
 
@@ -36,12 +35,6 @@ def _as_str(token) -> str:
     return token.decode() if isinstance(token, bytes) else token
 
 
-def _sasl_xoauth2(user: str, token: str) -> str:
-    """build the base64 XOAUTH2 SASL initial-response string"""
-    raw = f"user={user}\x01auth=Bearer {token}\x01\x01".encode()
-    return base64.b64encode(raw).decode()
-
-
 class OAuth2Auth:
     """XOAUTH2 auth: a static `token`, or a `token_provider` (sync/async callable) fetched fresh at connect time"""
 
@@ -71,18 +64,6 @@ class OAuth2Auth:
         token = await self._resolve_token()
         # mail.xoauth2(user, token) f-string-interpolates token, so it MUST be str -
         # bytes would interpolate the b'...' repr and corrupt the Bearer value.
-        xoauth2 = getattr(mail, "xoauth2", None)
-        if xoauth2 is not None:
-            result, data = await xoauth2(self.user, token)
-        elif hasattr(mail, "authenticate"):
-            # escape hatch for a non-aioimaplib client (aioimaplib's IMAP4 always
-            # has .xoauth2, never .authenticate); untested against any real driver
-            result, data = await mail.authenticate(
-                "XOAUTH2", lambda _: _sasl_xoauth2(self.user, token)
-            )
-        else:
-            raise RuntimeError(
-                "IMAP client exposes no XOAUTH2 entrypoint (no .xoauth2 or .authenticate)"
-            )
+        result, data = await mail.xoauth2(self.user, token)
         if result != "OK":
             raise RuntimeError(f"xoauth2 auth failed: {result} {data}")