Correct ClientSession's exception-delivery docstring; list shared.session removal

The class docstring claimed transport exceptions reach message_handler
only when the session builds its own dispatcher, but a caller-supplied
JSONRPCDispatcher with no stream-exception hook is wired up the same
way (the standard Client path), so exceptions arrive on any stream-backed
dispatcher. Say that instead. Also add mcp.shared.session and its
RequestResponder stub to the removed-import-paths bullet in whats-new.

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

Author: maxisbey

docs/whats-new.md

@@ -146,7 +146,7 @@ Each of these is a section in the **[Migration Guide](migration.md)**:
 
 * The **WebSocket transport**, both sides, and the `mcp[ws]` extra. It was never part of the MCP specification.
 * The **experimental Tasks** API (`mcp.*.experimental`). 2026-07-28 moves tasks out of the core protocol and into an official extension ([SEP-2663](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2663)), which this SDK does not implement yet.
-* `mcp.types`, `mcp.shared.version`, and `mcp.shared.progress` as import paths.
+* `mcp.types`, `mcp.shared.version`, `mcp.shared.progress`, and `mcp.shared.session` (with the `RequestResponder` stub v1 `message_handler` annotations imported) as import paths.
 * The deprecated `streamablehttp_client` spelling, and the `get_session_id` callback from `streamable_http_client` (which now yields exactly two streams).
 * `McpError`, renamed **`MCPError`** with a direct `(code, message, data)` constructor.
 * `MCPServer.get_context()`, `mount_path=`, and the lowlevel `Server`'s decorator methods, ContextVar, and handler dicts.

src/mcp/client/session.py

@@ -334,8 +334,10 @@ class ClientSession:
     `dispatcher=`), enter as an async context manager, then call
     `initialize()`. The dispatcher owns the receive loop and request
     correlation; this class owns the typed MCP layer and the constructor
-    callbacks. Transport `Exception` items reach `message_handler` only when
-    the session builds its own dispatcher from a stream pair.
+    callbacks. Transport `Exception` items reach `message_handler` on any
+    stream-backed dispatcher (`JSONRPCDispatcher`), whether built here from a
+    stream pair or supplied without a stream-exception hook of its own; an
+    in-process `DirectDispatcher` carries none.
 
     Extension `result_claims` fold into tools/call parsing at `adopt()`;
     `notification_bindings` observe vendor notifications via bounded FIFOs.