[codex] Add managed new-thread model settings (#29683)

## Why

Admins need persistent defaults for the model, reasoning effort, and
service tier shown when the Desktop App creates a new thread. These are
initialization defaults rather than runtime constraints: the App should
use them to initialize its draft while still allowing a user to make an
explicit selection.

The app-server therefore needs to expose the managed values before
thread creation without changing `thread/start` behavior for other
clients.

## What changed

- Parse `model`, `model_reasoning_effort`, and `service_tier` from
`[models.new_thread]` in `requirements.toml`.
- Compose the `models` requirements through the existing
requirements-layer precedence rules.
- Expose the resolved values through `configRequirements/read` as
`requirements.models.newThread`.
- Add the corresponding app-server protocol types and regenerate the
JSON and TypeScript schema fixtures.
- Document the new `configRequirements/read` fields in the app-server
README.

## Scope

This PR is data plumbing only. It does not apply these values during
`thread/start` and does not change thread creation for existing
app-server clients, resumed or forked sessions, internal or subagent
sessions, `codex exec`, or the TUI. A companion Desktop App change owns
draft initialization, sends the effective settings for ordinary and
prewarmed starts, and preserves explicit user changes.

## Validation

- Requirements deserialization coverage for `[models.new_thread]`
- Requirements-layer precedence coverage
- App-server API mapping coverage
- `configRequirements/read` integration coverage
- Regenerated app-server JSON and TypeScript schema fixtures

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -1753,6 +1753,7 @@ fn config_requirements_granular_allowed_approval_policy_is_marked_experimental()
             hooks: None,
             enforce_residency: None,
             network: None,
+            models: None,
         });
 
     assert_eq!(reason, Some("askForApproval.granular"));