Make the protocol transport-agnostic and use tokio-tungstenite

Replace the hand-rolled WebSocket implementation with a transport
boundary (Frame over any Sink/Stream) plus adapters for tokio-tungstenite
(server/client features) and axum (axum feature). The JSON wire format
is unchanged, so existing peers keep working.

Fixes:
- unbounded frame lengths were allocated up front; the server now
  enforces message/frame size limits (1 MiB default)
- responses arriving before `request` subscribed were lost
- an accept() error ended `session_loop`
- requests sent right after connecting could arrive before handlers
  were registered; the receiver now starts after `on_conn` returns
- panics in the receive loop skipped `on_close` and leaked sessions;
  `on_close` now runs exactly once and handler panics fail only their
  request
- unknown methods and invalid data got no reply; they now get an
  error response
- slow handlers blocked pongs and responses

Adds `on_notification`, `request_timeout`, `closed`, `is_closed`, `id`,
`ServerConfig`, `from_transport`, `from_tungstenite` and `from_axum`,
integration tests, and an axum example. Bumps to 0.2.0 since
`Session::connect` now takes a URL and the `ws` module is removed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

examples/client.rs

@@ -1,5 +1,5 @@
 use serde::{Deserialize, Serialize};
-use session_rs::{Method, session::Session};
+use session_rs::{Method, Session};
 
 #[derive(Debug, Serialize, Deserialize)]
 struct Data;
@@ -13,7 +13,7 @@ impl Method for Data {
 
 #[tokio::main(flavor = "current_thread")]
 async fn main() -> session_rs::Result<()> {
-    let session = Session::connect("127.0.0.1:8080", "/").await?;
+    let session = Session::connect("ws://127.0.0.1:8080").await?;
 
     session.start_receiver();
 
@@ -31,8 +31,6 @@ async fn main() -> session_rs::Result<()> {
             .await?
     );
 
-    tokio::time::sleep(tokio::time::Duration::from_millis(1000)).await;
-
     session.close().await?;
     Ok(())
 }