devtools: Fix getBreakpointListActor handler in watcher actor (#38624)

the Firefox devtools client often sends multiple
`getBreakpointListActor` requests to the watcher actor when connecting,
and the Firefox devtools server returns the same breakpoint list actor
every time, but Servo returns a new breakpoint list actor each time.
this patch aligns Servo’s behaviour with Firefox.

Testing: this patch adds a devtools test

Signed-off-by: Delan Azabani <dazabani@igalia.com>
Co-authored-by: atbrakhi <atbrakhi@igalia.com>

python/servo/devtools_tests.py

@@ -57,6 +57,16 @@ class DevtoolsTests(unittest.IsolatedAsyncioTestCase):
         self.web_servers = None
         self.web_server_threads = None
 
+    # Watcher tests
+
+    def test_watcher_returns_same_breakpoint_list_actor_every_time(self):
+        self.run_servoshell(url="data:text/html,")
+        devtools = self._setup_devtools_client()
+        response1 = devtools.watcher.get_breakpoint_list_actor()
+        response2 = devtools.watcher.get_breakpoint_list_actor()
+        self.assertEqual(response1["breakpointList"]["actor"], response2["breakpointList"]["actor"])
+
+    # Sources list
     # Classic script vs module script:
     # - <https://html.spec.whatwg.org/multipage/#classic-script>
     # - <https://html.spec.whatwg.org/multipage/#module-script>
@@ -66,8 +76,6 @@ class DevtoolsTests(unittest.IsolatedAsyncioTestCase):
     # Non-worker(?) script sources can be inline, external, or blob.
     # Worker script sources can be external or blob.
 
-    # Sources list
-
     def test_sources_list(self):
         self.start_web_server(test_dir=os.path.join(DevtoolsTests.script_path, "devtools_tests/sources"))
         self.run_servoshell()