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

docs/OAUTH_SUPPORT.md

@@ -9,8 +9,8 @@ This document describes the OAuth 2.1 authorization implementation for Model Con
 - Authorization server metadata discovery
 - Dynamic client registration
 - Automatic token refresh
-- Authorized SSE transport implementation
 - Authorized HTTP Client implementation
+
 ## Usage Guide
 
 ### 1. Enable Features
@@ -19,7 +19,7 @@ Enable the auth feature in Cargo.toml:
 
 ```toml
 [dependencies]
-rmcp = { version = "0.1", features = ["auth", "transport-sse-client"] }
+rmcp = { version = "0.1", features = ["auth", "transport-streamable-http-client-reqwest"] }
 ```
 
 ### 2. Use OAuthState
@@ -42,29 +42,27 @@ rmcp = { version = "0.1", features = ["auth", "transport-sse-client"] }
     // Get authorization URL and guide user to open it
     let auth_url = oauth_state.get_authorization_url().await?;
     println!("Please open the following URL in your browser for authorization:\n{}", auth_url);
-    
+
     // Handle callback - In real applications, this is typically done in a callback server
     let auth_code = "Authorization code (`code` param) obtained from browser after user authorization";
     let csrf_token = "CSRF token (`state` param) obtained from browser after user authorization";
     let credentials = oauth_state.handle_callback(auth_code, csrf_token).await?;
-    
+
     println!("Authorization successful, access token: {}", credentials.access_token);
 
 ```
 
-### 4. Use Authorized SSE Transport and create client
+### 4. Use Authorized Streamable HTTP Transport and create client
 
 ```rust ignore
-    let transport =
-        match create_authorized_transport(MCP_SSE_URL.to_string(), oauth_state, Some(retry_config))
-            .await
-        {
-            Ok(t) => t,
-            Err(e) => {
-                tracing::error!("Failed to create authorized transport: {}", e);
-                return Err(anyhow::anyhow!("Connection failed: {}", e));
-            }
-        };
+    let am = oauth_state
+        .into_authorization_manager()
+        .ok_or_else(|| anyhow::anyhow!("Failed to get authorization manager"))?;
+    let client = AuthClient::new(reqwest::Client::default(), am);
+    let transport = StreamableHttpClientTransport::with_client(
+        client,
+        StreamableHttpClientTransportConfig::with_uri(MCP_SERVER_URL),
+    );
 
     // Create client and connect to MCP server
     let client_service = ClientInfo::default();
@@ -77,20 +75,19 @@ rmcp = { version = "0.1", features = ["auth", "transport-sse-client"] }
     let client = oauth_state.to_authorized_http_client().await?;
 ```
 
-## Complete Example
-client: Please refer to `examples/clients/src/auth/oauth_client.rs` for a complete usage example.
-server: Please refer to `examples/servers/src/complex_auth_sse.rs` for a complete usage example.
-### Running the Example in server
-```bash
-# Run example
-cargo run --example complex_auth_sse
-```
+## Complete Examples
 
-### Running the Example in client
+- **Client**: `examples/clients/src/auth/oauth_client.rs`
+- **Server**: `examples/servers/src/complex_auth_streamhttp.rs`
+
+### Running the Examples
 
 ```bash
-# Run example
-cargo run --example oauth-client
+# Run the OAuth server
+cargo run --example servers_complex_auth_streamhttp
+
+# Run the OAuth client (in another terminal)
+cargo run --example clients_oauth_client
 ```
 
 ## Authorization Flow Description
@@ -123,4 +120,4 @@ If you encounter authorization issues, check the following:
 - [MCP Authorization Specification](https://spec.modelcontextprotocol.io/specification/2025-03-26/basic/authorization/)
 - [OAuth 2.1 Specification Draft](https://oauth.net/2.1/)
 - [RFC 8414: OAuth 2.0 Authorization Server Metadata](https://datatracker.ietf.org/doc/html/rfc8414)
-- [RFC 7591: OAuth 2.0 Dynamic Client Registration Protocol](https://datatracker.ietf.org/doc/html/rfc7591) 
+- [RFC 7591: OAuth 2.0 Dynamic Client Registration Protocol](https://datatracker.ietf.org/doc/html/rfc7591)