[Web] Handle LocalSession init in WASM RPC server (#18687)

## Why

When a client connects without session_constructor_args, the RPC
protocol sends nargs=0 (LocalSession request). The WASM RPC server
requires ["rpc.WasmSession", wasm_binary] instead.
             
## How
Adds handling to skip LocalSession init packets and wait for the proper
WasmSession init, with a log message for debugging.

Author: guan404ming

web/src/rpc_server.ts

@@ -228,6 +228,16 @@ export class RPCServer {
       // eslint-disable-next-line @typescript-eslint/no-unused-vars
       const ver = Uint8ArrayToString(reader.readByteArray());
       const nargs = reader.readU32();
+
+      // nargs=0 means no session_constructor_args (LocalSession request).
+      // WASM RPC requires ["rpc.WasmSession", wasm_binary]. Wait for proper init.
+      if (nargs === 0) {
+        this.log("Received LocalSession init (nargs=0), waiting for WasmSession init...");
+        this.requestBytes(SizeOf.I64);
+        this.state = RPCServerState.ReceivePacketHeader;
+        return;
+      }
+
       const args = [];
       for (let i = 0; i < nargs; ++i) {
         const typeIndex = reader.readU32();