Qualify message_handler delivery docs; fix stale README line

IncomingMessage's docstring and the callbacks page said every server
notification reaches message_handler, but notifications/cancelled is
applied by the dispatcher and a subscription ack for a live listen()
stream is consumed by that stream, so neither is delivered. Say so.
Also drop the "shared type aliases" mention from the interaction
suite's layout guide now that _helpers.py holds only the recording
transport.

No-Verification-Needed: comment- and doc-only change

Author: maxisbey

docs/client/callbacks.md

@@ -135,7 +135,7 @@ Two more. Neither declares anything.
 
 `logging_callback` receives every `notifications/message` a server sends, as `LoggingMessageNotificationParams` (`level`, `logger`, `data`). Protocol logging is itself deprecated by the 2026-07-28 spec (**[Logging](../handlers/logging.md)** has what to do instead), so this callback exists for the servers that still emit it.
 
-`message_handler` is the catch-all: every server notification reaches it (as well as its specific callback), and on a stream-backed transport so does every transport-level `Exception`. Annotate its parameter with `IncomingMessage` (`ServerNotification | Exception`, exported from `mcp.client`). The one pattern worth knowing is `if isinstance(message, Exception): raise message`, so a broken connection fails loudly instead of vanishing.
+`message_handler` is the catch-all: every server notification the session surfaces reaches it (as well as its specific callback), and on a stream-backed transport so does every transport-level `Exception`. Two never do: `notifications/cancelled` is applied by the SDK rather than surfaced, and a subscription acknowledgment for a live `listen()` stream is consumed by that stream. Annotate the parameter with `IncomingMessage` (`ServerNotification | Exception`, exported from `mcp.client`). The one pattern worth knowing is `if isinstance(message, Exception): raise message`, so a broken connection fails loudly instead of vanishing.
 
 ## Recap
 

src/mcp/client/session.py

@@ -172,7 +172,12 @@ async def __call__(self, params: types.LoggingMessageNotificationParams) -> None
 
 
 IncomingMessage: TypeAlias = types.ServerNotification | Exception
-"""What `message_handler` receives: every server notification, plus transport-level exceptions."""
+"""What `message_handler` receives: the server notifications the session surfaces, plus transport-level exceptions.
+
+`notifications/cancelled` is applied by the dispatcher and never surfaced, and a
+`notifications/subscriptions/acknowledged` for a live `listen()` stream is consumed by that
+stream, so neither reaches the handler.
+"""
 
 
 class MessageHandlerFnT(Protocol):

tests/interaction/README.md

@@ -40,7 +40,7 @@ flows — with a single subprocess test for stdio.
 ```text
 tests/interaction/
   _requirements.py      the requirements manifest (see below)
-  _helpers.py           shared type aliases + the wire-recording transport
+  _helpers.py           the wire-recording transport
   _connect.py           the transport-parametrized connection factories
   conftest.py           the connect fixture (the transport matrix)
   test_coverage.py      enforces the manifest ↔ test contract