fix(rmcp): import base64-only prompt content types under the feature

[ragen1337]

ImageContent and AudioContent are only used by new_image/new_audio, which are behind the base64 feature, so building without it gives an unused-imports warning:

cargo check -p rmcp --no-default-features --features server,client
warning: unused imports: `AudioContent` and `ImageContent`

Moved the imports into those two functions (same as the base64 imports there) and added them to the test module. No API changes.

Checked with clippy --all-features -D warnings, the prompt tests and fmt.

[chrikrah]

@ragen1337 the warning is real and this removes it. Approve.

non-blocking: no CI job will ever show the difference. Both clippy steps in .github/workflows/ci.yml:62-71 enable base64. The second gets it from the cargo metadata feature list, so -D warnings never sees prompt.rs built without it. A registry or git consumer misses it too, since cargo hands dependencies --cap-lints allow. Who sees it is whoever builds rmcp itself with the feature off, in-tree or as a path dependency. Clear it and CI can one day gate that feature set.

Verification

git archive of 49e3cae into a clean directory, rustc 1.96.1 from the pinned rust-toolchain.toml.

$ cargo check -p rmcp --no-default-features --message-format short
crates/rmcp/src/model.rs:322:19: warning: methods `numeric_string_value` and `matches_response_id` are never used
crates/rmcp/src/model.rs:657:11: warning: associated items `TRANSPORT_CLOSED_MARKER`, ... are never used
warning: `rmcp` (lib) generated 2 warnings
# both of these predate the branch

# crates/rmcp/src/model/prompt.rs restored to ae2f9c9, nothing else touched
$ cargo check -p rmcp --no-default-features --message-format short
crates/rmcp/src/model/prompt.rs:5:15: warning: unused imports: `AudioContent` and `ImageContent`
warning: `rmcp` (lib) generated 3 warnings (run `cargo fix --lib -p rmcp` to apply 1 suggestion)

$ cargo check -p rmcp --all-targets --no-default-features --message-format short
crates/rmcp/tests/test_prompt_routers.rs:74:13: error[E0425]: cannot find function `async_function_prompt_attr` in this scope
error: could not compile `rmcp` (test "test_prompt_routers") due to 19 previous errors
error: could not compile `rmcp` (lib test) due to 4 previous errors
# not run: the test suite, and any build on Windows or macOS

@DaleSeo, a third clippy step would hold this, and it is not green yet. --all-targets does not compile without default features. --lib alone still trips -D warnings on the two model.rs warnings above. Worth gating that feature set after those two?

[ragen1337]

Thanks for checking.

The two model.rs ones are only used from service/transport, so they're dead without client or server. Default features have the same issue on main: matches_response_id, set_client_request_metadata, is_authorization_required.

I have a fix that adds #[cfg]s, same as send_subscription_request. With this PR and #1318, RUSTFLAGS="-D warnings" cargo test -p rmcp --lib --no-default-features passes with it and fails without. @DaleSeo want a separate PR?

[DaleSeo]

The MSRV failure isn't from your change. It was fixed on main in #1316. Could you rebase onto the latest main and push? A re-run alone won't pick up the fix.

[ragen1337]

Rebased onto latest main (includes #1316) and pushed. The workflows are waiting for approval to run on the fork PR. Thanks!

[DaleSeo]

Thanks for the fix, @ragen1337!