Fix embedded FletApp stdout/stderr dropped by the framed transport

The Pyodide↔Dart postMessage transport frames every packet as [type:u8][payload]
(0x00 = MsgPack Flet protocol frame, 0x01 = raw DataChannel frame), and
PyodideConnection.send_message prepends the 0x00 byte. The python_output shim,
however, posted the raw msgpack [7, {...}] with no type byte, so the Dart side
read msgpack's leading 0x92 (array-of-2 marker) as an unknown packet type and
silently dropped every stdout/stderr line — the host page's Console pane never
saw embedded-app output. Prepend the 0x00 type byte in both worker templates
(client + cookiecutter build template).

Author: FeodorFitsner

client/web/python-worker.js

@@ -170,8 +170,15 @@ self.initPyodide = async function () {
             import msgpack as _msgpack
 
         def _send_python_output(text, is_stderr):
+            # Frame exactly like PyodideConnection.send_message: a 0x00 type
+            # byte (0x00 = MsgPack Flet protocol frame, 0x01 = raw DataChannel
+            # frame) in front of the packed [action, body]. Without the prefix
+            # the Dart side reads msgpack's leading 0x92 as an unknown packet
+            # type and silently drops the line. bytes([0]) avoids a literal
+            # NUL escape inside this JS template string.
             flet_js.receive_callback(
-                _msgpack.packb(
+                bytes([0])
+                + _msgpack.packb(
                     [7, {"text": text, "is_stderr": bool(is_stderr)}]
                 )
             )

sdk/python/templates/build/{{cookiecutter.out_dir}}/web/python-worker.js

@@ -170,8 +170,15 @@ self.initPyodide = async function () {
             import msgpack as _msgpack
 
         def _send_python_output(text, is_stderr):
+            # Frame exactly like PyodideConnection.send_message: a 0x00 type
+            # byte (0x00 = MsgPack Flet protocol frame, 0x01 = raw DataChannel
+            # frame) in front of the packed [action, body]. Without the prefix
+            # the Dart side reads msgpack's leading 0x92 as an unknown packet
+            # type and silently drops the line. bytes([0]) avoids a literal
+            # NUL escape inside this JS template string.
             flet_js.receive_callback(
-                _msgpack.packb(
+                bytes([0])
+                + _msgpack.packb(
                     [7, {"text": text, "is_stderr": bool(is_stderr)}]
                 )
             )