feat(gateway): add trusted-proxy auth mode (#15940)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 279d4b304f
Co-authored-by: nickytonline <833231+nickytonline@users.noreply.github.com>
Co-authored-by: steipete <58493+steipete@users.noreply.github.com>
Reviewed-by: @steipete

src/gateway/protocol/schema/snapshot.ts

@@ -52,6 +52,14 @@ export const SnapshotSchema = Type.Object(
     configPath: Type.Optional(NonEmptyString),
     stateDir: Type.Optional(NonEmptyString),
     sessionDefaults: Type.Optional(SessionDefaultsSchema),
+    authMode: Type.Optional(
+      Type.Union([
+        Type.Literal("none"),
+        Type.Literal("token"),
+        Type.Literal("password"),
+        Type.Literal("trusted-proxy"),
+      ]),
+    ),
   },
   { additionalProperties: false },
 );