feat: Add Copilot 3rd-party agent metrics

[Tens1des]

Adds totals_by_3rd_party_agent to Copilot usage metrics report structs.

GitHub changelog: https://github.blog/changelog/2026-08-07-copilot-usage-metrics-api-adds-agent-app-activity/

• New CopilotMetricsThirdPartyAgent type (agent_name, agent_id, user_initiated_interaction_count, optional session_count)
• Field wired on CopilotDailyMetrics, CopilotUserDailyMetrics, and CopilotUserPeriodicMetrics
• Tests for daily and user-daily downloads; accessors regenerated via script/generate.sh

Fixes #4459

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.55%. Comparing base (27edd67) to head (ba5c8e5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4460   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files         194      194           
  Lines       19884    19884           
=======================================
  Hits        19398    19398           
  Misses        268      268           
  Partials      218      218           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[gmlewis]

Thank you, @Tens1des!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra