feat(app-server): expose usage report [3 of 4]

[fcoury-oai]

Why

The TUI should read usage through the app-server boundary instead of reaching into state directly.

What Changed

• Adds the usage/read app-server request.
• Maps state usage reports into v2 app-server protocol response types.
• Regenerates JSON and TypeScript schema fixtures.

How to Test

Targeted tests:

• just write-app-server-schema
• cargo test -p codex-app-server-protocol
• cargo check -p codex-app-server

Stack

1. #24121 - Usage storage
2. #24122 - Usage attribution
3. #24123 - App-server usage API (this PR)
4. #24124 - TUI /usage command