[verified] fix(mcp-oauth): bridge httpx auth_flow bidirectional generator

HermesMCPOAuthProvider.async_auth_flow wrapped the SDK's auth_flow with
'async for item in super().async_auth_flow(request): yield item', which
discards httpx's .asend(response) values and resumes the inner generator
with None. This broke every OAuth MCP server on the first HTTP response
with 'NoneType' object has no attribute 'status_code' crashing at
mcp/client/auth/oauth2.py:505.

Replace with a manual bridge that forwards .asend() values into the
inner generator, preserving httpx's bidirectional auth_flow contract.

Add tests/tools/test_mcp_oauth_bidirectional.py with two regression
tests that drive the flow through real .asend() round-trips. These
catch the bug at the unit level; prior tests only exercised
_initialize() and disk-watching, never the full generator protocol.

Verified against BetterStack MCP:
  Before: 'Connection failed (11564ms): NoneType...' after 3 retries
  After:  'Connected (2416ms); Tools discovered: 83'

Regression from #11383.

tools/mcp_oauth_manager.py

@@ -125,9 +125,28 @@ def _make_hermes_provider_class() -> Optional[type]:
                     self._hermes_server_name, exc,
                 )
 
-            # Delegate to the SDK's auth flow
-            async for item in super().async_auth_flow(request):
-                yield item
+            # Manually bridge the bidirectional generator protocol. httpx's
+            # auth_flow driver (httpx._client._send_handling_auth) calls
+            # ``auth_flow.asend(response)`` to feed HTTP responses back into
+            # the generator. A naive wrapper using ``async for item in inner:
+            # yield item`` DISCARDS those .asend(response) values and resumes
+            # the inner generator with None, so the SDK's
+            # ``response = yield request`` branch in
+            # mcp/client/auth/oauth2.py sees response=None and crashes at
+            # ``if response.status_code == 401`` with AttributeError.
+            #
+            # The bridge below forwards each .asend() value into the inner
+            # generator via inner.asend(incoming), preserving the bidirectional
+            # contract. Regression from PR #11383 caught by
+            # tests/tools/test_mcp_oauth_bidirectional.py.
+            inner = super().async_auth_flow(request)
+            try:
+                outgoing = await inner.__anext__()
+                while True:
+                    incoming = yield outgoing
+                    outgoing = await inner.asend(incoming)
+            except StopAsyncIteration:
+                return
 
     return HermesMCPOAuthProvider