Add proper regression test for OAuth retry performance bug

This test validates the fix for PR #1206 which resolved a critical
performance issue where every request was being sent twice due to
incorrect indentation of retry logic.

The test:
- Simulates a successful authenticated request (200 response)
- Counts how many times the request is yielded
- FAILS with buggy version: 2 yields (double sending)
- PASSES with fix: 1 yield (correct behavior)

This directly tests the root cause that made OAuth requests 2x slower
and ensures this regression cannot occur again.

Author: felixweinberger

tests/client/test_auth.py

@@ -698,6 +698,47 @@ async def test_auth_flow_with_no_tokens(self, oauth_provider, mock_storage):
         assert oauth_provider.context.current_tokens.access_token == "new_access_token"
         assert oauth_provider.context.token_expiry_time is not None
 
+    @pytest.mark.anyio
+    async def test_auth_flow_no_unnecessary_retry_after_oauth(self, oauth_provider, mock_storage, valid_tokens):
+        """Test that requests are not retried unnecessarily - the core bug that caused 2x performance degradation."""
+        # Pre-store valid tokens so no OAuth flow is needed
+        await mock_storage.set_tokens(valid_tokens)
+        oauth_provider.context.current_tokens = valid_tokens
+        oauth_provider.context.token_expiry_time = time.time() + 1800
+        oauth_provider._initialized = True
+
+        test_request = httpx.Request("GET", "https://api.example.com/mcp")
+        auth_flow = oauth_provider.async_auth_flow(test_request)
+
+        # Count how many times the request is yielded
+        request_yields = 0
+
+        # First request - should have auth header already
+        request = await auth_flow.__anext__()
+        request_yields += 1
+        assert request.headers["Authorization"] == "Bearer test_access_token"
+
+        # Send a successful 200 response
+        response = httpx.Response(200, request=request)
+        
+        # In the buggy version, this would yield the request AGAIN unconditionally
+        # In the fixed version, this should end the generator
+        try:
+            extra_request = await auth_flow.asend(response)
+            request_yields += 1
+            # If we reach here, the bug is present
+            pytest.fail(
+                f"Unnecessary retry detected! Request was yielded {request_yields} times. "
+                f"This indicates the retry logic bug that caused 2x performance degradation. "
+                f"The request should only be yielded once for successful responses."
+            )
+        except StopAsyncIteration:
+            # This is the expected behavior - no unnecessary retry
+            pass
+
+        # Verify exactly one request was yielded (no double-sending)
+        assert request_yields == 1, f"Expected 1 request yield, got {request_yields}"
+
 
 @pytest.mark.parametrize(
     (