feat: allow server to tell transport the protocol version

Author: mattzcarey

src/server/index.ts

@@ -442,6 +442,12 @@ export class Server<
 
         const protocolVersion = SUPPORTED_PROTOCOL_VERSIONS.includes(requestedVersion) ? requestedVersion : LATEST_PROTOCOL_VERSION;
 
+        // Inform the transport of the negotiated protocol version.
+        // This allows the transport to validate subsequent request headers.
+        if (this.transport?.setProtocolVersion) {
+            this.transport.setProtocolVersion(protocolVersion);
+        }
+
         return {
             protocolVersion,
             capabilities: this.getCapabilities(),

src/shared/transport.ts

@@ -122,7 +122,14 @@ export interface Transport {
     sessionId?: string;
 
     /**
-     * Sets the protocol version used for the connection (called when the initialize response is received).
+     * Sets the protocol version after negotiation during initialization.
+     * This is called by the Server/Client class after the initialize handshake completes.
      */
     setProtocolVersion?: (version: string) => void;
+
+    /**
+     * Gets the negotiated protocol version, if set.
+     * Available after initialization completes.
+     */
+    protocolVersion?: string;
 }