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/rig-integration/src/config/mcp.rs

@@ -17,9 +17,6 @@ pub enum McpServerTransportConfig {
     Streamable {
         url: String,
     },
-    Sse {
-        url: String,
-    },
     Stdio {
         command: String,
         #[serde(default)]
@@ -68,10 +65,6 @@ impl McpServerTransportConfig {
                     rmcp::transport::StreamableHttpClientTransport::from_uri(url.to_string());
                 ().serve(transport).await?
             }
-            McpServerTransportConfig::Sse { url } => {
-                let transport = rmcp::transport::SseClientTransport::start(url.to_string()).await?;
-                ().serve(transport).await?
-            }
             McpServerTransportConfig::Stdio {
                 command,
                 args,