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/Cargo.toml

@@ -9,7 +9,6 @@ rmcp = { workspace = true, features = [
     "server",
     "macros",
     "client",
-    "transport-sse-server",
     "transport-io",
     "transport-streamable-http-server",
     "auth",
@@ -54,14 +53,6 @@ tokio-util = { version = "0.7", features = ["codec"] }
 name = "servers_counter_stdio"
 path = "src/counter_stdio.rs"
 
-[[example]]
-name = "servers_counter_sse"
-path = "src/counter_sse.rs"
-
-[[example]]
-name = "servers_counter_sse_directly"
-path = "src/counter_sse_directly.rs"
-
 [[example]]
 name = "servers_memory_stdio"
 path = "src/memory_stdio.rs"
@@ -70,14 +61,6 @@ path = "src/memory_stdio.rs"
 name = "servers_counter_streamhttp"
 path = "src/counter_streamhttp.rs"
 
-[[example]]
-name = "servers_complex_auth_sse"
-path = "src/complex_auth_sse.rs"
-
-[[example]]
-name = "servers_simple_auth_sse"
-path = "src/simple_auth_sse.rs"
-
 [[example]]
 name = "servers_prompt_stdio"
 path = "src/prompt_stdio.rs"
@@ -105,3 +88,11 @@ path = "src/completion_stdio.rs"
 [[example]]
 name = "servers_progress_demo"
 path = "src/progress_demo.rs"
+
+[[example]]
+name = "servers_simple_auth_streamhttp"
+path = "src/simple_auth_streamhttp.rs"
+
+[[example]]
+name = "servers_complex_auth_streamhttp"
+path = "src/complex_auth_streamhttp.rs"