feat!: remove SSE transport support (#562)

SSE transport has been removed from the MCP specification in favor of
streamable HTTP. This removes all SSE-specific transport code:

- Remove `transport-sse-client` and `transport-sse-server` features
- Remove `SseClientTransport` and `SseServer` types
- Remove SSE-specific examples (`counter_sse`, `counter_sse_directly`)
- Migrate auth examples from SSE to streamable HTTP
- Update tests to remove SSE transport usage
- Update documentation

BREAKING CHANGE: The following have been removed:
- `transport-sse-client` feature
- `transport-sse-client-reqwest` feature
- `transport-sse-server` feature
- `SseClientTransport` type
- `SseServer` type
- `sse_client` and `sse_server` modules

Users should migrate to streamable HTTP transport which provides
equivalent functionality. See `StreamableHttpClientTransport` and
`StreamableHttpService` for the replacement APIs.

Ref: https://github.com/modelcontextprotocol/rust-sdk/pull/561#issuecomment-3576551699

crates/rmcp/tests/test_with_js.rs

@@ -2,7 +2,7 @@ use rmcp::{
     ServiceExt,
     service::QuitReason,
     transport::{
-        ConfigureCommandExt, SseServer, StreamableHttpClientTransport, StreamableHttpServerConfig,
+        ConfigureCommandExt, StreamableHttpClientTransport, StreamableHttpServerConfig,
         TokioChildProcess,
         streamable_http_server::{
             session::local::LocalSessionManager, tower::StreamableHttpService,
@@ -14,42 +14,11 @@ use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 mod common;
 use common::calculator::Calculator;
 
-const SSE_BIND_ADDRESS: &str = "127.0.0.1:8000";
 const STREAMABLE_HTTP_BIND_ADDRESS: &str = "127.0.0.1:8001";
 const STREAMABLE_HTTP_JS_BIND_ADDRESS: &str = "127.0.0.1:8002";
 
 #[tokio::test]
-async fn test_with_js_client() -> anyhow::Result<()> {
-    let _ = tracing_subscriber::registry()
-        .with(
-            tracing_subscriber::EnvFilter::try_from_default_env()
-                .unwrap_or_else(|_| "debug".to_string().into()),
-        )
-        .with(tracing_subscriber::fmt::layer())
-        .try_init();
-    tokio::process::Command::new("npm")
-        .arg("install")
-        .current_dir("tests/test_with_js")
-        .spawn()?
-        .wait()
-        .await?;
-
-    let ct = SseServer::serve(SSE_BIND_ADDRESS.parse()?)
-        .await?
-        .with_service(Calculator::default);
-
-    let exit_status = tokio::process::Command::new("node")
-        .arg("tests/test_with_js/client.js")
-        .spawn()?
-        .wait()
-        .await?;
-    assert!(exit_status.success());
-    ct.cancel();
-    Ok(())
-}
-
-#[tokio::test]
-async fn test_with_js_server() -> anyhow::Result<()> {
+async fn test_with_js_stdio_server() -> anyhow::Result<()> {
     let _ = tracing_subscriber::registry()
         .with(
             tracing_subscriber::EnvFilter::try_from_default_env()