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

examples/clients/src/auth/oauth_client.rs

@@ -11,9 +11,9 @@ use rmcp::{
     ServiceExt,
     model::ClientInfo,
     transport::{
-        SseClientTransport,
+        StreamableHttpClientTransport,
         auth::{AuthClient, OAuthState},
-        sse_client::SseClientConfig,
+        streamable_http_client::StreamableHttpClientTransportConfig,
     },
 };
 use serde::Deserialize;
@@ -25,7 +25,6 @@ use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
 const MCP_SERVER_URL: &str = "http://localhost:3000/mcp";
 const MCP_REDIRECT_URI: &str = "http://localhost:8080/callback";
-const MCP_SSE_URL: &str = "http://localhost:3000/mcp/sse";
 const CALLBACK_PORT: u16 = 8080;
 const CALLBACK_HTML: &str = include_str!("callback.html");
 
@@ -150,14 +149,10 @@ async fn main() -> Result<()> {
         .into_authorization_manager()
         .ok_or_else(|| anyhow::anyhow!("Failed to get authorization manager"))?;
     let client = AuthClient::new(reqwest::Client::default(), am);
-    let transport = SseClientTransport::start_with_client(
+    let transport = StreamableHttpClientTransport::with_client(
         client,
-        SseClientConfig {
-            sse_endpoint: MCP_SSE_URL.into(),
-            ..Default::default()
-        },
-    )
-    .await?;
+        StreamableHttpClientTransportConfig::with_uri(MCP_SERVER_URL),
+    );
 
     // Create client and connect to MCP server
     let client_service = ClientInfo::default();