Add a cacheable Bazel app-server schema bundle (#42174)

## What changed

- Add a `schema_bundle` rule that generates stable and experimental app-server
  schema directories as declared Bazel actions.
- Include the pinned `zstd` executable in the bundle so consumers can normalize
  generated JSON without a separate `bazel run` invocation.
- Expose the app-server protocol test binary as the schema generator and add the
  manual `//bazel/schema:public-schema-bundle` target.

GitOrigin-RevId: 0a7ce47258d297dbf20db78626f2843145ee1b49

codex-rs/app-server-protocol/BUILD.bazel

@@ -9,3 +9,11 @@ codex_rust_crate(
         allow_empty = True,
     ),
 )
+
+alias(
+    name = "schema-generator",
+    testonly = True,
+    actual = ":app-server-protocol-unit-tests-bin",
+    tags = ["manual"],
+    visibility = ["//bazel/schema:__pkg__"],
+)