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]>

Cargo.toml

@@ -1,21 +1,38 @@
 [package]
 name = "session-rs"
-version = "0.1.3"
+version = "0.2.0"
 edition = "2024"
-description = "A lightweight async WebSocket protocol"
+description = "A lightweight async request/response and notification protocol over WebSockets"
 license = "Apache-2.0"
+repository = "https://git.selimaj.dev/selimaj-dev/session-rs"
+
+[features]
+default = ["server", "client"]
+# `SessionServer`: a standalone tokio-tungstenite WebSocket server.
+server = ["dep:tokio-tungstenite", "tokio/net"]
+# `Session::connect`: a tokio-tungstenite WebSocket client (ws:// only without a TLS feature).
+client = ["dep:tokio-tungstenite", "tokio-tungstenite/connect"]
+# `wss://` support for the client.
+rustls = ["client", "tokio-tungstenite/rustls-tls-webpki-roots"]
+native-tls = ["client", "tokio-tungstenite/native-tls"]
+# `Session::from_axum`: run a session on an axum WebSocket upgrade.
+axum = ["dep:axum"]
 
 [dependencies]
-base64 = "0.22.1"
-rand = "0.10.0"
-serde = { version = "1.0.228", features = ["serde_derive"] }
+futures-util = { version = "0.3.34", default-features = false, features = ["sink", "std"] }
+serde = { version = "1.0.228", features = ["derive"] }
 serde_json = "1.0.149"
-sha1 = "0.10.6"
-tokio = { version = "1.49.0", features = [
-    "io-util",
-    "macros",
-    "net",
-    "rt",
-    "sync",
-    "time",
-] }
+tokio = { version = "1.49.0", features = ["macros", "rt", "sync", "time"] }
+tokio-tungstenite = { version = "0.30.0", default-features = false, features = ["handshake"], optional = true }
+axum = { version = "0.8.9", default-features = false, features = ["ws"], optional = true }
+
+[dev-dependencies]
+tokio = { version = "1.49.0", features = ["macros", "rt-multi-thread", "net", "time", "io-util"] }
+axum = { version = "0.8.9", features = ["ws"] }
+
+[[example]]
+name = "axum"
+required-features = ["axum"]
+
+[package.metadata.docs.rs]
+all-features = true