Stabilize protocol schema fixture generation (#13886)

## What changed
- TypeScript schema fixture generation now goes through in-memory tree
helpers rather than a heavier on-disk generation path.
- The comparison logic normalizes generated banner and path differences
that are not semantically relevant to the exported schema.
- TypeScript and JSON fixture coverage are split into separate tests,
and the expensive schema-export tests are serialized in `nextest`.

## Why this fixes the flake
- The original fixture coverage mixed several heavy codegen paths into
one monolithic test and then compared generated output that included
incidental banner/path differences.
- On Windows CI, that combination created both runtime pressure and
output variance unrelated to the schema shapes we actually care about.
- Splitting the coverage isolates failures by format, in-memory
generation reduces filesystem churn, normalization strips generator
noise, and serializing the heavy tests removes parallel resource
contention.

## Scope
- Production helper change plus test changes.

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

@@ -17,6 +17,9 @@ pub use protocol::thread_history::*;
 pub use protocol::v1::*;
 pub use protocol::v2::*;
 pub use schema_fixtures::SchemaFixtureOptions;
+#[doc(hidden)]
+pub use schema_fixtures::generate_typescript_schema_fixture_subtree_for_tests;
+pub use schema_fixtures::read_schema_fixture_subtree;
 pub use schema_fixtures::read_schema_fixture_tree;
 pub use schema_fixtures::write_schema_fixtures;
 pub use schema_fixtures::write_schema_fixtures_with_options;