Remove unused StreamableHTTPTransport.get_session_id()

The method backed the get_session_id callback that streamable_http_client
used to yield as the third tuple element. That callback was removed in v2,
leaving the getter with no callers and no way to reach the transport
instance through the public entry point. Drop it along with the stale TODO
comments, and note the removal in the existing migration-guide section.

Author: maxisbey

docs/migration.md

@@ -2110,7 +2110,7 @@ Client-side stream resumption is also unchanged: the transport reconnects a drop
 
 The `get_session_id` callback (third element of the returned tuple) has been removed from `streamable_http_client`. The function now returns a 2-tuple `(read_stream, write_stream)` instead of a 3-tuple.
 
-The `GetSessionIdCallback` type alias is gone as well, so `from mcp.client.streamable_http import GetSessionIdCallback` now raises `ImportError`. Drop the annotation, or inline `Callable[[], str | None]` if your own wrapper code still needs the type.
+The `GetSessionIdCallback` type alias is gone as well, so `from mcp.client.streamable_http import GetSessionIdCallback` now raises `ImportError`. Drop the annotation, or inline `Callable[[], str | None]` if your own wrapper code still needs the type. The `StreamableHTTPTransport.get_session_id()` method that backed the callback is removed too.
 
 If you need to capture the session ID (e.g., for session resumption testing), you can use httpx2 event hooks to capture it from the response headers:
 

src/mcp/client/streamable_http.py

@@ -635,15 +635,7 @@ async def terminate_session(self, client: httpx2.AsyncClient) -> None:
         except Exception as exc:  # pragma: no cover
             logger.warning(f"Session termination failed: {exc}")
 
-    # TODO(Marcelo): Check the TODO below, and cover this with tests if necessary.
-    def get_session_id(self) -> str | None:
-        """Get the current session ID."""
-        return self.session_id  # pragma: no cover
 
-
-# TODO(Marcelo): I've dropped the `get_session_id` callback because it breaks the Transport protocol. Is that needed?
-# It's a completely wrong abstraction, so removal is a good idea. But if we need the client to find the session ID,
-# we should think about a better way to do it. I believe we can achieve it with other means.
 @asynccontextmanager
 async def streamable_http_client(
     url: str,