fix: remove redirect_policy from McpHttpClientFactory Protocol

The Protocol describes the caller-side contract. No caller passes
redirect_policy to the factory (sse_client only passes headers, timeout,
auth). Adding it would break downstream code implementing custom factories.

Github-Issue: #2106

Author: dgenio

src/mcp/shared/_httpx_utils.py

@@ -85,7 +85,6 @@ def __call__(  # pragma: no branch
         headers: dict[str, str] | None = None,
         timeout: httpx.Timeout | None = None,
         auth: httpx.Auth | None = None,
-        redirect_policy: RedirectPolicy = RedirectPolicy.BLOCK_SCHEME_DOWNGRADE,
     ) -> httpx.AsyncClient: ...