Copilot usage metrics: missing ai_credits_used and AI adoption phase fields

[Tens1des]

Description

The Copilot usage metrics report JSON includes fields that go-github does not yet unmarshal:

• ai_credits_used on per-user reports (*-users-1-day, *-users-28-day)
• ai_adoption_phase on per-user reports
• totals_by_ai_adoption_phase on aggregated enterprise/organization daily reports

These fields are dropped by json.Unmarshal today.

References

• https://github.blog/changelog/2026-06-19-ai-credits-consumed-per-user-now-in-the-copilot-usage-metrics-api/
• https://github.blog/changelog/2026-05-29-copilot-usage-metrics-api-adds-cohorts-for-ai-adoption/
• https://docs.github.com/en/copilot/reference/copilot-usage-metrics/copilot-usage-metrics

Proposed fix

Add matching fields/types on CopilotUserDailyMetrics, CopilotUserPeriodicMetrics, and CopilotDailyMetrics, with download-path tests (same pattern as #4460).

[Tens1des]

I'll take this