docs(thread): fully qualify Sphinx Chat refs (gemini review)

Rewrites `:py:meth:`Chat.on_subscribed_message`` and
`:py:meth:`Chat.on_direct_message`` in `Thread.subscribe`'s docstring as
`:py:meth:`~chat_sdk.chat.Chat.on_subscribed_message`` and
`:py:meth:`~chat_sdk.chat.Chat.on_direct_message``.

`Chat` is intentionally not imported in `thread.py` (TYPE_CHECKING-only) to
avoid a circular import. The unqualified refs wouldn't resolve under Sphinx;
the absolute paths do, and the `~` keeps the rendered link text short.

The `test_thread_subscribe_docstring_documents_dm_carveout` pin still passes:
its `"on_direct_message" in doc` assertion matches the substring inside the
new qualified ref.

https://claude.ai/code/session_01FyMxQn2BEAzmwKS1GZczKj

Author: claude

src/chat_sdk/thread.py

@@ -507,8 +507,8 @@ async def subscribe(self) -> None:
         """Subscribe to future messages in this thread.
 
         Once subscribed, messages in non-DM threads trigger
-        :py:meth:`Chat.on_subscribed_message` handlers. DM threads route to
-        :py:meth:`Chat.on_direct_message` first when a direct message handler
+        :py:meth:`~chat_sdk.chat.Chat.on_subscribed_message` handlers. DM threads route to
+        :py:meth:`~chat_sdk.chat.Chat.on_direct_message` first when a direct message handler
         is registered. The initial message that triggered subscription will
         NOT fire the handler.
         """