Address review finding: declare tokio's net feature

Review finding:

This line adds tokio's `sync` feature for the new Tower server's `oneshot::channel`, but the same new binary also calls `tokio::net::TcpListener::bind` in `scenarios/http/rust/tower/opentelemetry-instrumentation-tower/server/src/main.rs`, and `net` is not in the declared feature list. `tokio::net` is gated behind tokio's `net` feature, so the crate compiles only because axum 0.8.9's `tokio` feature (enabled two lines above) transitively turns on `tokio/net`. That makes a direct API use depend on another dependency's feature choice. A fix is to add `net` to the tokio feature list so the feature the code uses directly is declared where it is used.

Analysis: Tokio exports `TcpListener` inside `cfg_net!`, so `tokio::net` exists only when the `net` feature is on. The workspace declaration listed `macros`, `rt-multi-thread` and `sync`, and axum 0.8.9's `tokio` feature turns on `tokio/net` for the whole build, which is what let the Tower server compile. Listing `net` beside `sync` states the feature the scenario binary uses directly. Cargo resolves the same versions, so `Cargo.lock` is unchanged.

Upsides: The Tower server no longer depends on axum's feature list to reach `tokio::net`, so dropping or narrowing the axum dependency fails at the manifest rather than deep in a compile error. The declaration now matches the same rule the `sync` entry follows.

Downsides: No material downside identified.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: trask

scenarios/http/rust/Cargo.toml

@@ -31,7 +31,7 @@ opentelemetry-otlp = { version = "0.32.0", default-features = false, features =
 opentelemetry_sdk = { version = "0.32.0", features = ["metrics", "rt-tokio", "trace"] }
 serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.145"
-tokio = { version = "1.48.0", features = ["macros", "rt-multi-thread", "sync"] }
+tokio = { version = "1.48.0", features = ["macros", "net", "rt-multi-thread", "sync"] }
 
 otel-conformance-http = { path = "../../../tools/http/test-client/rust" }
 otel-conformance-scenario = { path = "../../../tools/rust/scenario" }