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/src/transport/auth.rs

@@ -74,7 +74,7 @@ impl CredentialStore for InMemoryCredentialStore {
     }
 }
 
-/// sse client with oauth2 authorization
+/// HTTP client with OAuth 2.0 authorization
 #[derive(Clone)]
 pub struct AuthClient<C> {
     pub http_client: C,
@@ -91,7 +91,7 @@ impl<C: std::fmt::Debug> std::fmt::Debug for AuthClient<C> {
 }
 
 impl<C> AuthClient<C> {
-    /// create new authorized sse client
+    /// Create a new authorized HTTP client
     pub fn new(http_client: C, auth_manager: AuthorizationManager) -> Self {
         Self {
             http_client,