Summary

Marks two API surfaces as deprecated on the v1.x line. Both are already removed on main (v2): the WebSocket transport in #2785 and experimental tasks support in #2714. This change gives 1.x users advance notice before they upgrade to 2.0.

• WebSocket transport — websocket_client and websocket_server now carry typing_extensions.deprecated markers and emit a DeprecationWarning when called. WebSocket was never part of the MCP specification; the streamable HTTP transport is the replacement.
• Experimental tasks entry points — ClientSession.experimental, the experimental_task_handlers constructor kwarg, ServerSession.experimental, and Server.experimental warn the same way. Tasks (SEP-1686) were removed from the MCP specification and are expected to return as a separate MCP extension in the future.

Design notes

• Warnings fire only on use. There are no import-time warnings, so libraries that import these modules at top level (without calling them) are unaffected at runtime.
• Library internals never trigger the warning. Internal callers now go through a private ServerSession._experimental accessor, and a regression test asserts that a plain client/server round trip emits zero deprecation warnings. @deprecated is applied outside @asynccontextmanager so the warning points at the caller's frame rather than contextlib internals.
• The type-checker signal is intentional. Editors and type checkers that flag deprecated symbols (pyright's reportDeprecated, ty's deprecated rule) will now surface these usages downstream — that static channel reaches maintainers more reliably than runtime warnings, which Python hides by default.
• Test-suite warning policy preserved. The suites that intentionally exercise these APIs get scoped filterwarnings ignores (a conftest for the tasks suite, a pytestmark for the websocket suite) instead of repo-global ignores, so the global filterwarnings = ["error"] net still catches any accidental warning from SDK internals elsewhere.
• The experimental tasks docs pages now show a deprecation banner.

Deliberately not in this PR: the task types in mcp.types and the experimental module classes are untouched. Runtime warnings there would fire from inside libraries that build on these types and spam their users; a follow-up can add static-only markers (@deprecated(..., category=None)) for those.

Testing

• New tests assert each entry point emits the warning, and that plain (non-tasks, non-websocket) usage stays warning-free.
• Full suite passes with 100% coverage; pyright strict is clean.

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