refactor: remove unused axum dependency from server-side-http feature

[andrewgazelka]

Summary

The server-side-http feature included dep:axum but axum was never actually used in the rmcp library source code (verified: 0 references found via grep).

The StreamableHttpService is a tower service that works with any HTTP server framework. Users can choose to use:

• axum (via Router::nest_service() or fallback_service())
• hyper directly (via hyper_util::service::TowerToHyperService)
• any other tower-compatible HTTP server

Motivation

This came up while integrating rmcp into nushell (PR #17161) - a reviewer raised concerns about adding axum as a dependency. After investigation, we found that:

1. StreamableHttpService only uses tower_service::Service trait
2. axum is listed as a dependency but has 0 actual usages in rmcp source
3. Examples that use axum already have their own explicit dependency

Changes

• Remove dep:axum from the server-side-http feature in crates/rmcp/Cargo.toml

Testing

• cargo build -p rmcp --features "transport-streamable-http-server" ✓
• cargo test -p rmcp --all-features ✓
• Examples still work (they have their own axum dependency)

Impact

Users who want to use axum can add it as a direct dependency (as the examples do). Users who prefer hyper or another tower-compatible server no longer have axum as a transitive dependency.

[ofek]

I was actually about to open a PR but didn't realize the dependency is unused. If we aren't removing this dependency definition, could you incorporate my would-be change?

Suggested change

	axum = { version = "0.8", default-features = false, features = ["http1", "tokio"], optional = true }

[alexhancock]

@andrewgazelka@ofek Yeah something like this makes sense.

It looks like it might only be used in crates/rmcp/tests?

[andrewgazelka]

🦾: Thanks for the feedback! 2a29c55

After looking closer, I realized axum is only used in the tests (not the library source itself), so I've moved it to dev-dependencies instead with the minimal features @ofek suggested:

[dev-dependencies]
axum = { version = "0.8", default-features = false, features = ["http1", "tokio"] }

This way:

• Library users don't get axum as a transitive dependency at all
• rmcp's own tests still work (they use axum to spin up test servers)
• Examples are unaffected (they have their own axum dependency)

Verified:

• cargo build -p rmcp --features "transport-streamable-http-server" ✓
• cargo test -p rmcp --all-features ✓
• cargo build -p mcp-server-examples --examples ✓
• cargo build -p mcp-client-examples --examples ✓

[andrewgazelka]

any thoughts on adding proper CI to rust-sdk as well? I suppose could have merged without realizing it was used in a test.

[ofek]

Thanks a lot!