refactor: extract EgressApplicator base class shared between backends

Pulls the duplicated apply_routes_change / validate_routes_content /
_routes_path logic into EgressApplicator (ABC) in backend/egress_apply.py.
DockerEgressApplicator and MacOSContainerEgressApplicator override the
single abstract _signal_bundle_reload method with their respective kill
commands. Module-level shims preserve the existing public API.

bot_bottle/backend/egress_apply.py

@@ -0,0 +1,50 @@
+"""Shared base class for host-side egress apply across backends.
+
+Each backend subclasses EgressApplicator and overrides _signal_bundle_reload
+with the backend-specific kill command.
+"""
+
+from __future__ import annotations
+
+from abc import ABC, abstractmethod
+from pathlib import Path
+
+from ..bottle_state import egress_state_dir
+from ..egress import EGRESS_ROUTES_FILENAME
+from ..egress_addon_core import load_routes
+
+
+class EgressApplyError(RuntimeError):
+    pass
+
+
+class EgressApplicator(ABC):
+    def apply_routes_change(self, slug: str, content: str) -> tuple[str, str]:
+        """Persist `content` to the live routes file and reload egress."""
+        self.validate_routes_content(content)
+        routes_path = self._routes_path(slug)
+        routes_path.parent.mkdir(parents=True, exist_ok=True)
+        before = routes_path.read_text(encoding="utf-8") if routes_path.exists() else ""
+        routes_path.write_text(content, encoding="utf-8")
+        routes_path.chmod(0o600)
+        self._signal_bundle_reload(slug)
+        return before, content
+
+    @staticmethod
+    def validate_routes_content(content: str) -> None:
+        try:
+            load_routes(content)
+        except ValueError as e:
+            raise EgressApplyError(
+                f"proposed routes.yaml is not valid: {e}"
+            ) from e
+
+    @staticmethod
+    def _routes_path(slug: str) -> Path:
+        return egress_state_dir(slug) / EGRESS_ROUTES_FILENAME
+
+    @abstractmethod
+    def _signal_bundle_reload(self, slug: str) -> None: ...
+
+
+__all__ = ["EgressApplicator", "EgressApplyError"]