`tool` macro expands to `serde_json` without re-export

[jonaro00]

Describe the bug
tool macro expands to serde_json here which causes compilation error in crates that don't have it as a dependency.

To Reproduce
Steps to reproduce the behavior:

1. cargo new asdf
2. cd asdf
3. cargo add rcmp -F macros
4. add basic tool call to src/main.rs

struct A;

#[rmcp::tool_router]
impl A {
    #[rmcp::tool]
    async fn foo() {}
}

5. error[E0433]: failed to resolve: use of unresolved module or unlinked crate serde_json

Proposed solution
Other location in the macro already assumes the downstream crate depends on rmcp, so expanding the macro to use the re-exported ::rmcp::serde_json::... should work.