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/common.rs

@@ -1,7 +1,4 @@
-#[cfg(any(
-    feature = "transport-streamable-http-server",
-    feature = "transport-sse-server"
-))]
+#[cfg(feature = "transport-streamable-http-server")]
 pub mod server_side_http;
 
 pub mod http_header;
@@ -10,6 +7,9 @@ pub mod http_header;
 #[cfg_attr(docsrs, doc(cfg(feature = "reqwest")))]
 mod reqwest;
 
+// Note: This module provides SSE stream parsing and auto-reconnect utilities.
+// It's used by the streamable HTTP client (which receives SSE-formatted responses),
+// not the removed SSE transport. The name is historical.
 #[cfg(feature = "client-side-sse")]
 #[cfg_attr(docsrs, doc(cfg(feature = "client-side-sse")))]
 pub mod client_side_sse;