Replace API deferred queue with efficient message batching system (#9012)

tests/integration/test_host_mode_many_entities.py

@@ -0,0 +1,57 @@
+"""Integration test for many entities to test API batching."""
+
+from __future__ import annotations
+
+import asyncio
+
+from aioesphomeapi import EntityState
+import pytest
+
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_host_mode_many_entities(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test API batching with many entities of different types."""
+    # Write, compile and run the ESPHome device, then connect to API
+    loop = asyncio.get_running_loop()
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        # Subscribe to state changes
+        states: dict[int, EntityState] = {}
+        entity_count_future: asyncio.Future[int] = loop.create_future()
+
+        def on_state(state: EntityState) -> None:
+            states[state.key] = state
+            # When we have received states from a good number of entities, resolve the future
+            if len(states) >= 50 and not entity_count_future.done():
+                entity_count_future.set_result(len(states))
+
+        client.subscribe_states(on_state)
+
+        # Wait for states from at least 50 entities with timeout
+        try:
+            entity_count = await asyncio.wait_for(entity_count_future, timeout=10.0)
+        except asyncio.TimeoutError:
+            pytest.fail(
+                f"Did not receive states from at least 50 entities within 10 seconds. "
+                f"Received {len(states)} states: {list(states.keys())}"
+            )
+
+        # Verify we received a good number of entity states
+        assert entity_count >= 50, f"Expected at least 50 entities, got {entity_count}"
+        assert len(states) >= 50, f"Expected at least 50 states, got {len(states)}"
+
+        # Verify we have different entity types by checking some expected values
+        sensor_states = [
+            s
+            for s in states.values()
+            if hasattr(s, "state") and isinstance(s.state, float)
+        ]
+
+        assert len(sensor_states) >= 50, (
+            f"Expected at least 50 sensor states, got {len(sensor_states)}"
+        )