Align request-body limit with the MCP SDK default

The middleware default was an arbitrary 10 MiB, above the 4 MiB the SDK
already enforces, so it never changed which requests were accepted.
Alias mcp.DefaultMaxRequestBodyBytes instead, making the earlier
enforcement point behaviour-preserving by construction.

Also pass the effective limit to StreamableHTTPOptions. Previously the
SDK kept its own 4 MiB default, so a larger configured
MaxRequestBodyBytes was silently capped; both layers now agree.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>