Add ChatGPT device login E2E coverage

[beiwei30]

Summary

• add a real CLI loopback E2E for the ChatGPT device-code flow
• verify flushed instructions, 404/403 pending handling, one-second polling, and the final authorization-code exchange
• verify 0600 credential persistence, restart-visible active status, no extra status requests, and secret redaction
• extend the shared auth harness with request timestamps and configurable device authorization values

Verification

• cargo test -p orbcode --test chatgpt_auth_device_e2e
• cargo test -p orbcode-config openai_oauth::tests::device
• cargo test -p orbcode-config chatgpt_credentials_round_trip
• cargo fmt --all --check
• cargo clippy -p orbcode --test chatgpt_auth_device_e2e -- -D warnings
• cargo test -p orbcode --test chatgpt_auth_browser_e2e
• cargo test -p orbcode --test chatgpt_auth_harness_e2e
• git diff --check

Mutation checks

The focused E2E was confirmed to fail when instruction flushing was removed, 404 pending became terminal, polling delay was removed, or the device callback URI was changed.

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