fix(rmcp): gate client- and server-only helpers behind their features

[ragen1337]

Follow-up to my comment in #1314.

Some crate-internal helpers are only called from client or server code, so building rmcp with fewer features fails under -D warnings:

RUSTFLAGS="-D warnings" cargo check -p rmcp --lib --no-default-features
error: methods `numeric_string_value` and `matches_response_id` are never used
error: associated items `TRANSPORT_CLOSED_MARKER`, `is_transport_closed`, and `transport_closed_token` are never used

Default, client-only and server-only builds fail the same way on other helpers. I gated them with #[cfg], like send_subscription_request. ErrorData::transport_closed is only used by the streamable HTTP client, so it now also needs client or server.

CI didn't see it because both clippy steps enable client and server, and the no-default-features test job from #1318 doesn't use -D warnings. @chrikrah suggested a clippy step for this in #1314, so I added one. It's --lib only because some tests don't build with reduced features.

With -D warnings, no features and every single feature fail on main and pass here. Clippy and fmt pass. cargo test --all-features has the same 2 child_process failures as main for me. No API changes.

[DaleSeo]

Thanks for the following up, @ragen1337! I left a couple of minor comments.