Add fixture-based tests for provider usage parsing

[rain2day]

Goal

Add tests around Codex and Claude usage parsing without using real credentials or network calls.

Why

Provider usage endpoints may change. Parser fixtures would make maintenance safer and help contributors avoid breaking Codex-only, Claude-only, and combined modes.

Scope

• Extract parser logic or add a small test harness that can run from CI.
• Use sanitized JSON fixtures only.
• Cover reset times, session usage, weekly usage, plan labels, and combined provider fallback.

Done When

• CI can run parser checks without local credentials.
• Docs mention where sanitized fixtures live.