fix(egress): emit preserve_auth into the generated routes

The flag from #453 was parsed off the manifest and honored by the
addon, but `_route_to_yaml_fields` never wrote it, so it was dropped on
the way to the proxy and has never worked end to end. Every registry
route silently kept the unconditional Authorization strip.

That is the whole of the Docker Hub / GHCR "unauthorized" blocker in
#392: podman fetched a valid 2658-byte bearer from auth.docker.io, the
proxy stripped it, and registry-1.docker.io answered the resulting
anonymous request with a fresh WWW-Authenticate challenge — which reads
exactly like a credential problem rather than a serializer gap.

Found by diffing a bottle's rendered routes.yaml against its manifest:
preserve_auth was in the manifest and absent from the output.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_egress.py

@@ -379,6 +379,25 @@ class TestRenderRoutes(unittest.TestCase):
         addon_routes = load_config(rendered).routes
         self.assertTrue(addon_routes[0].git_fetch)
 
+    def test_preserve_auth_round_trips_to_the_addon(self):
+        """Regression: the manifest parsed preserve_auth and the addon honored
+        it, but the renderer in between dropped it — so the flag never reached
+        the proxy and registry pulls kept failing with "unauthorized" while
+        the config looked correct everywhere it was inspected."""
+        from bot_bottle.egress_addon_core import load_config
+        b = _bottle([{"host": "registry-1.docker.io", "preserve_auth": True}])
+        routes = egress_routes_for_bottle(b)
+        rendered = egress_render_routes(routes)
+        self.assertIn("preserve_auth: true", rendered)
+        self.assertTrue(load_config(rendered).routes[0].preserve_auth)
+
+    def test_preserve_auth_omitted_when_unset(self):
+        b = _bottle([{"host": "x.example"}])
+        rendered = egress_render_routes(egress_routes_for_bottle(b))
+        self.assertNotIn("preserve_auth", rendered)
+        from bot_bottle.egress_addon_core import load_config
+        self.assertFalse(load_config(rendered).routes[0].preserve_auth)
+
     def test_log_zero_omitted_from_render(self):
         b = _bottle([{"host": "x.example"}])
         routes = egress_routes_for_bottle(b)