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/clients/Cargo.toml

@@ -9,7 +9,6 @@ publish = false
 [dependencies]
 rmcp = { workspace = true, features = [
     "client",
-    "transport-sse-client-reqwest",
     "reqwest",
     "transport-streamable-http-client-reqwest",
     "transport-child-process",
@@ -30,10 +29,6 @@ axum = "0.8"
 reqwest = "0.12"
 clap = { version = "4.0", features = ["derive"] }
 
-[[example]]
-name = "clients_sse"
-path = "src/sse.rs"
-
 [[example]]
 name = "clients_git_stdio"
 path = "src/git_stdio.rs"