The rmcp lib test target does not build without the macros feature

[chrikrah]

object! is #[macro_export]ed and documented from model.rs:42, but model.rs:60 puts it behind the macros feature, so it is unreachable in a --no-default-features build. The lib test target is where that shows on 8f9a28e: four unit tests call the macro.

$ cd /tmp/rmcp-main && cargo check -p rmcp --lib --no-default-features --profile test
# rustc 1.96.1 (31fca3adb 2026-06-26), the pinned rust-toolchain.toml, on 8f9a28e
error: cannot find macro `object` in this scope
    --> crates/rmcp/src/model.rs:5529:17
note: `object` is imported here, but it is a function, not a macro
    --> crates/rmcp/src/model.rs:4837:9
error: could not compile `rmcp` (lib test) due to 4 previous errors   # :5459 :5488 :5503 :5529

$ cargo check -p rmcp --lib --no-default-features --features macros --profile test
    Finished `test` profile [unoptimized + debuginfo] target(s) in 7.63s

# same export, `#[cfg(feature = "macros")]` deleted from model.rs:60, nothing else touched
$ cargo test -p rmcp --lib --no-default-features
    test result: ok. 139 passed; 0 failed; 0 ignored
# not run: the integration targets under crates/rmcp/tests, and any host other than Linux

The expansion is serde_json::json! around the ungated model::object function, serde_json is a plain dependency, and macros is dep:rmcp-macros + dep:pastey. Deleting the gate is the one-line option. Keeping it means object(json!({ ... })) at the four call sites, or a #[cfg(feature = "macros")] on the four #[test] functions.

Dropping --lib fails too, in 13 integration targets, on missing server/client items. Different cause, and none of the above fixes it.

@DaleSeo, the feature-matrix job on feat/rationalize-feature-flags (#1067) runs cargo check, which never builds a test target, and ci.yml:268 and :302 both enable macros. Would --profile test on that job, or a cargo test -p rmcp --lib --no-default-features line beside it, be the right home for this?