feat: vendor app-server protocol schema fixtures (#10371)

Similar to what @sayan-oai did in openai/codex#8956 for
`config.schema.json`, this PR updates the repo so that it includes the
output of `codex app-server generate-json-schema` and `codex app-server
generate-ts` and adds a test to verify it is in sync with the current
code.

Motivation:
- This makes any schema changes introduced by a PR transparent during
code review.
- In particular, this should help us catch PRs that would introduce a
non-backwards-compatible change to the app schema (eventually, this
should also be enforced by tooling).
- Once https://github.com/openai/codex/pull/10231 is in to formalize the
notion of "experimental" fields, we can work on ensuring the
non-experimental bits are backwards-compatible.

`codex-rs/app-server-protocol/tests/schema_fixtures.rs` was added as the
test and `just write-app-server-schema` can be use to generate the
vendored schema files.

Incidentally, when I run:

```
rg _ codex-rs/app-server-protocol/schema/typescript/v2
```

I see a number of `snake_case` names that should be `camelCase`.

codex-rs/app-server-protocol/src/lib.rs

@@ -1,6 +1,7 @@
 mod export;
 mod jsonrpc_lite;
 mod protocol;
+mod schema_fixtures;
 
 pub use export::generate_json;
 pub use export::generate_ts;
@@ -10,3 +11,5 @@ pub use protocol::common::*;
 pub use protocol::thread_history::*;
 pub use protocol::v1::*;
 pub use protocol::v2::*;
+pub use schema_fixtures::read_schema_fixture_tree;
+pub use schema_fixtures::write_schema_fixtures;