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/Cargo.toml

@@ -33,7 +33,7 @@ schemars = { version = "1.0", optional = true, features = ["chrono04"] }
 # for image encoding
 base64 = { version = "0.22", optional = true }
 
-# for SSE client
+# for HTTP client
 reqwest = { version = "0.12", default-features = false, features = [
   "json",
   "stream",
@@ -99,20 +99,16 @@ server-side-http = [
   "dep:sse-stream",
   "tower",
 ]
-# SSE client
-client-side-sse = ["dep:sse-stream", "dep:http"]
-
-transport-sse-client = ["client-side-sse", "transport-worker"]
-transport-sse-client-reqwest = ["transport-sse-client", "reqwest"]
 
 transport-worker = ["dep:tokio-stream"]
 
+# SSE stream parsing utilities (used by streamable HTTP client for SSE-formatted responses)
+client-side-sse = ["dep:sse-stream", "dep:http"]
 
 # Streamable HTTP client
 transport-streamable-http-client = ["client-side-sse", "transport-worker"]
 transport-streamable-http-client-reqwest = ["transport-streamable-http-client", "reqwest"]
 
-
 transport-async-rw = ["tokio/io-util", "tokio-util/codec"]
 transport-io = ["transport-async-rw", "tokio/io-std"]
 transport-child-process = [
@@ -120,12 +116,6 @@ transport-child-process = [
   "tokio/process",
   "dep:process-wrap",
 ]
-transport-sse-server = [
-  "transport-async-rw",
-  "transport-worker",
-  "server-side-http",
-  "dep:axum",
-]
 transport-streamable-http-server = [
   "transport-streamable-http-server-session",
   "server-side-http",
@@ -162,8 +152,6 @@ required-features = [
   "reqwest",
   "server",
   "client",
-  "transport-sse-server",
-  "transport-sse-client",
   "transport-child-process",
 ]
 path = "tests/test_with_python.rs"
@@ -173,7 +161,6 @@ name = "test_with_js"
 required-features = [
   "server",
   "client",
-  "transport-sse-server",
   "transport-child-process",
   "transport-streamable-http-server",
   "transport-streamable-http-client",