app-server-protocol: export flat v2 schema bundle (#13324)

## Summary
- add an `--experimental` flag to the export binary and thread the
option through TypeScript and JSON schema generation
- flatten the v2 schema bundle into a datamodel-code-generator-friendly
`codex_app_server_protocol.v2.schemas.json` export
- retarget shared helper refs to namespaced v2 definitions, add coverage
for the new export behavior, and vendor the generated schema fixtures

## Validation
- `cargo test -p codex-app-server-protocol` (71 unit tests and bin
targets passed locally; the final schema fixture integration target was
revalidated via fresh schema regeneration and a tree diff)
- `./target/debug/write_schema_fixtures --schema-root <tmpdir>`
- `diff -rq app-server-protocol/schema <tmpdir>`

## Tickets
- None

codex-rs/app-server-protocol/src/bin/export.rs

@@ -14,9 +14,21 @@ struct Args {
     /// Optional Prettier executable path to format generated TypeScript files
     #[arg(short = 'p', long = "prettier", value_name = "PRETTIER_BIN")]
     prettier: Option<PathBuf>,
+
+    /// Include experimental API methods and fields in generated output.
+    #[arg(long = "experimental")]
+    experimental: bool,
 }
 
 fn main() -> Result<()> {
     let args = Args::parse();
-    codex_app_server_protocol::generate_types(&args.out_dir, args.prettier.as_deref())
+    codex_app_server_protocol::generate_ts_with_options(
+        &args.out_dir,
+        args.prettier.as_deref(),
+        codex_app_server_protocol::GenerateTsOptions {
+            experimental_api: args.experimental,
+            ..codex_app_server_protocol::GenerateTsOptions::default()
+        },
+    )?;
+    codex_app_server_protocol::generate_json_with_experimental(&args.out_dir, args.experimental)
 }