fix: resolve all invalid-return-type ty diagnostics across codebase

Widen return type annotations to match actual control flow, add
unreachable assertions after retry loops ty cannot prove terminate,
split ambiguous union returns (auth.py credential pool), and remove
the AIOHTTP_AVAILABLE conditional-import guard from api_server.py.

gateway/platforms/qqbot/adapter.py

@@ -1842,6 +1842,7 @@ class QQAdapter(BasePlatformAdapter):
                     await asyncio.sleep(1.5 * (attempt + 1))
                 else:
                     raise
+        raise AssertionError("unreachable: retry loop exhausted")
 
     # Maximum time (seconds) to wait for reconnection before giving up on send.
     _RECONNECT_WAIT_SECONDS = 15.0