[Client] Expose session ID from HttpTransport (#497)

* Expose session ID from HttpTransport

* [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.

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Co-authored-by: Christopher Hertel <mail@christopher-hertel.de>

src/Client/Transport/HttpTransport.php

@@ -121,6 +121,22 @@ class HttpTransport extends BaseTransport implements HeaderAwareTransportInterfa
         $this->headerCallback = $callback;
     }
 
+    /**
+     * The session ID minted by the server for this connection, if any.
+     *
+     * 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()
+     */
+    public function getSessionId(): ?string
+    {
+        return $this->sessionId;
+    }
+
     public function send(string $data): void
     {
         $request = $this->requestFactory->createRequest('POST', $this->endpoint)