webext: Rewrite native app starting logic

Using asyncio.run is preferred by Python.
`asyncio.get_event_loop` raises a `RuntimeError` on my machine,
which means that there is no event loop set.
This is a change in 3.14, seems that we need to use `asyncio.set_event_loop` before running.
So I use `asyncio.run` directly to avoid this.

Handle `SIGTERM` signal so we can quit gracefully.
Firefox will send `SIGTERM` on *nix systems, and use Windows's way to kill the native app.
Not using Windows so not sure if `SIGTERM` will also be raised on Windows,
but I do not find a clean way to notify killing.

See also: https://docs.python.org/3.14/library/asyncio-eventloop.html#asyncio.get_event_loop
          https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging#closing_the_native_app

Author: arenekosreal

webext/app/credential_manager_shim.py

@@ -7,6 +7,7 @@
 from enum import Enum
 import json
 import logging
+import signal
 import struct
 import sys
 from typing import Optional
@@ -395,23 +396,26 @@ async def run(cmd, options, origin, top_origin):
         raise Exception(f"unknown cmd: {cmd}")
 
 
-logging.info("starting credential_manager_shim")
-while True:
-    logging.debug("starting event loop message")
-    receivedMessage = getMessage()
-    request_id = receivedMessage["requestId"]
-    try:
-        cmd = receivedMessage["cmd"]
-
-        options = None
-        if "options" in receivedMessage:
-            options = receivedMessage["options"]
-        origin = receivedMessage["origin"]
-        top_origin = receivedMessage["topOrigin"]
-        loop = asyncio.get_event_loop()
-        auth_data = loop.run_until_complete(run(cmd, options, origin, top_origin))
-        sendMessage(encodeMessage({"requestId": request_id, "data": auth_data}))
-    except Exception as e:
-        logging.error("Failed to send message", exc_info=e)
-        sendMessage(encodeMessage({"requestId": request_id, "error": str(e)}))
-        logging.debug("Sent error message")
+quit = asyncio.Event()
+
+async def main():
+    logging.info("starting credential_manager_shim")
+    while not quit.is_set():
+        logging.debug("starting event loop message")
+        receivedMessage = getMessage()
+        request_id = receivedMessage["requestId"]
+        try:
+            cmd = receivedMessage["cmd"]
+            options = receivedMessage.get("options", None)
+            origin = receivedMessage["origin"]
+            top_origin = receivedMessage["topOrigin"]
+            auth_data = await run(cmd, options, origin, top_origin)
+            sendMessage(encodeMessage({"requestId": request_id, "data": auth_data}))
+        except Exception as e:
+            logging.error("Failed to send message", exc_info=e)
+            sendMessage(encodeMessage({"requestId": request_id, "error": str(e)}))
+            logging.debug("Sent error message")
+    logging.info("quitting credential_manager_shim")
+
+signal.signal(signal.SIGTERM, lambda _, __ : quit.set())
+asyncio.run(main())