[Client] State what getSessionId() returns on 2026-07-28

That revision removed protocol-level sessions (SEP-2567), so the getter is always null there. Adds the changelog entry.

Author: chr-hertel

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.9.0
+-----
+
+* Add `HttpTransport::getSessionId()` to read the server-minted `Mcp-Session-Id`: a request-scoped caller can persist it and pass it back through the constructor's `$headers` on a later transport. Always `null` on `2026-07-28`, which removed protocol-level sessions.
+
 0.8.0
 -----
 

src/Client/Transport/HttpTransport.php

@@ -124,12 +124,11 @@ public function onHeaders(callable $callback): void
     /**
      * The session ID minted by the server for this connection, if any.
      *
-     * Captured from the Mcp-Session-Id response header of legacy-era stateful
-     * servers. Callers that cannot keep the transport alive between protocol
-     * interactions (request-scoped PHP runtimes, for example) can persist this
-     * value and pass it back through the constructor's $headers parameter on a
-     * later transport, which deliberately wins over the internally tracked
-     * value, to address the same server-side session again.
+     * Captured from the Mcp-Session-Id response header of a handshake-era
+     * server; always null on 2026-07-28, which removed protocol-level sessions
+     * (SEP-2567). A caller that cannot keep the transport alive between
+     * interactions can persist it and pass it back through the constructor's
+     * $headers on a later transport, where it wins over the tracked value.
      *
      * @return string|null null until the server mints a session, and again after close()
      */