[bluetooth_proxy] Add an advertisement filter hook (#19220)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: pre-commit-ci-lite[bot] <117423508+pre-commit-ci-lite[bot]@users.noreply.github.com>

tests/component_tests/bluetooth_proxy/test_advertisement_filter.py

@@ -0,0 +1,13 @@
+"""The codegen hook external filtering components use to turn on the filter slot."""
+
+from esphome.components import bluetooth_proxy
+from esphome.core import CORE
+
+
+def test_enable_advertisement_filter_emits_define() -> None:
+    """External components call this rather than emitting the define."""
+    bluetooth_proxy.enable_advertisement_filter()
+
+    assert "USE_BLUETOOTH_PROXY_ADVERTISEMENT_FILTER" in {
+        define.name for define in CORE.defines
+    }