feat(providers): support ChatGPT reset credits [LET-10670]

[jnjpng]

Summary

Adds the Letta Code service and listener-protocol portion of LET-10670.

• lists authoritative ChatGPT banked reset-credit inventory for local and API-backed OAuth providers
• consumes a selected reset credit with a required caller-generated idempotency key
• adds typed listener list/consume commands, validation, routing, and responses
• refreshes both usage and reset inventory immediately after reset or already_redeemed
• preserves state for nothing_to_reset and no_credit outcomes
• caches inventory briefly while supporting explicit forced refreshes
• sanitizes upstream errors and keeps OAuth credentials and raw response bodies out of listener messages and logs

Impact

Letta Code clients can use one protocol surface for local and Cloud-stored ChatGPT OAuth providers. This is the service foundation for a separate terminal UI PR.

The API-backed path depends on letta-cloud#14039. Existing ChatGPT usage reads remain unchanged for ineligible or disconnected providers.

Validation

• bun run check — all 12 checks passed
• focused listener and usage regression suite — 241 tests passed
• pre-commit TypeScript, architecture, formatting, and file-size checks passed