sandbox: register_service ownership check (Phase 2)

_handle_register_service now requires the service domain to be one this
group owns (same main-side _owned_domains() derivation as the fire_event
gate), so a compromised sandbox can no longer squat
persistent_notification.* or any unclaimed domain.service. Unowned
domains are rejected with a HomeAssistantError → remote-error frame; the
existing refuse-to-clobber-an-existing-handler check is kept.

Existing register_service tests updated to own their mock domains via a
MockConfigEntry(sandbox="built-in").

homeassistant/components/sandbox/bridge.py

@@ -520,6 +520,13 @@ class SandboxBridge:
         sandbox side. Exception translation reuses
         :func:`_translate_remote_error`.
 
+        The service ``domain`` must be one this group owns (same main-side
+        :meth:`_owned_domains` derivation the fire_event gate uses): a
+        compromised sandbox may not squat ``persistent_notification.*`` or any
+        other ``domain.service`` slot outside the domains main routed to it.
+        An unowned domain is rejected with a :class:`HomeAssistantError` (the
+        channel turns it into a remote-error frame).
+
         If a service with the same ``(domain, service)`` already exists
         on main (e.g. the host ``light`` EntityComponent registered
         ``light.turn_on`` for our proxy entities, or another integration
@@ -528,6 +535,18 @@ class SandboxBridge:
         """
         domain = msg.domain.lower()
         service = msg.service.lower()
+        if domain not in self._owned_domains():
+            _LOGGER.warning(
+                "SandboxBridge[%s]: refusing register_service for unowned "
+                "domain %r (%s.%s)",
+                self.group,
+                domain,
+                domain,
+                service,
+            )
+            raise HomeAssistantError(
+                f"register_service: domain {domain!r} not owned by group {self.group!r}"
+            )
         supports_response = _parse_supports_response(msg.supports_response)
         if self.hass.services.has_service(domain, service):
             _LOGGER.debug(

tests/components/sandbox/test_bridge.py

@@ -539,6 +539,9 @@ async def test_register_entity_auto_loads_domain_component(
 
 async def test_register_service_installs_forwarder(hass: HomeAssistant) -> None:
     """A sandbox-registered service appears on main and forwards calls back."""
+    MockConfigEntry(
+        domain="mirror_demo", title="Mirror", sandbox="built-in"
+    ).add_to_hass(hass)
     _bridge, main_channel, sandbox_channel = await _wire(hass)
     seen_calls: list[pb.CallService] = []
 
@@ -583,6 +586,9 @@ async def test_forwarded_context_restores_on_echoed_state(
     echoing that same context_id, main restores the *original*
     ``user_id`` / ``parent_id`` instead of minting a fresh attribution.
     """
+    MockConfigEntry(
+        domain="mirror_demo", title="Mirror", sandbox="built-in"
+    ).add_to_hass(hass)
     _bridge, main_channel, sandbox_channel = await _wire(hass)
     forwarded_ids: list[str] = []
 
@@ -653,6 +659,9 @@ async def test_register_service_skips_existing_handler(
     hass: HomeAssistant,
 ) -> None:
     """Main already owning ``(domain, service)`` is not clobbered."""
+    MockConfigEntry(
+        domain="mirror_local", title="Mirror", sandbox="built-in"
+    ).add_to_hass(hass)
     _bridge, main_channel, sandbox_channel = await _wire(hass)
 
     async def _local(_call: Any) -> None:
@@ -682,6 +691,9 @@ async def test_unregister_service_removes_forwarder(
     hass: HomeAssistant,
 ) -> None:
     """``unregister_service`` drops the bridge-installed forwarder."""
+    MockConfigEntry(
+        domain="mirror_demo", title="Mirror", sandbox="built-in"
+    ).add_to_hass(hass)
     _bridge, main_channel, sandbox_channel = await _wire(hass)
 
     try:

tests/components/sandbox/test_schema_and_unload.py

@@ -257,6 +257,9 @@ async def test_register_service_with_schema_validates_on_main(
     hass: HomeAssistant,
 ) -> None:
     """Sandbox-mirrored service uses its reconstructed schema on main calls."""
+    MockConfigEntry(domain="mock_svc", title="Mock", sandbox="built-in").add_to_hass(
+        hass
+    )
     main_channel, sandbox_channel = make_channel_pair(
         name_a="main-mock", name_b="sandbox-mock"
     )