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/servers/src/html/sse_auth_index.html

@@ -1,33 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-    <title>RMCP Authorized SSE Server</title>
-    <style>
-        body { font-family: Arial, sans-serif; margin: 40px; line-height: 1.6; }
-        h1 { color: #333; }
-        .endpoint { background: #f4f4f4; padding: 10px; border-radius: 5px; }
-        pre { background: #eee; padding: 10px; border-radius: 5px; }
-    </style>
-</head>
-<body>
-    <h1>RMCP Authorized SSE Server</h1>
-    <p>This is a Server-Sent Events server example that requires OAuth authorization.</p>
-    
-    <h2>Available Endpoints:</h2>
-    <ul>
-        <li class="endpoint"><code>/api/health</code> - Health check</li>
-        <li class="endpoint"><code>/api/token/{token_id}</code> - Get test token (available: demo, test)</li>
-        <li class="endpoint"><code>/sse</code> - SSE connection endpoint (requires authorization)</li>
-        <li class="endpoint"><code>/message</code> - Message sending endpoint (requires authorization)</li>
-    </ul>
-    
-    <h2>Usage:</h2>
-    <pre>
-        # Get a token
-        curl http://127.0.0.1:8000/api/token/demo
-
-        # Connect to SSE using the token
-        curl -H "Authorization: Bearer demo-token" http://127.0.0.1:8000/sse
-    </pre>
-</body>
-</html>