Banner for pending Copilot comments in agent review sessions

Summary

As we expand review experiences in the agent window (and begin using it for PR reviews), surface when there are pending Copilot comments for the current session by showing a banner with actions to "Resolve" or "View". This will help reviewers notice and act on Copilot suggestions and speed up PR review flow.

Motivation

- Low-friction improvement that reduces missed Copilot comments during reviews.
- Makes the agent review experience more discoverable and actionable for PRs.

Acceptance criteria

- A banner is shown in the agent review window when there are one or more pending Copilot comments for the session.
- Banner indicates the number of pending comments (e.g., "3 pending Copilot comments").
- Banner provides at least two actions:
- "View" — opens the comments panel/session focused on pending Copilot comments.
- "Resolve" — marks pending Copilot comments as resolved (or opens a confirmation flow).
- Banner persists until the comments are resolved or the session is explicitly dismissed.
- Banner is accessible (keyboard + screen reader) and non-blocking.
- Behavior is consistent when used in PR review contexts (agent window invoked during PR review).

Notes / open questions

- Should "Resolve" resolve all comments in the session automatically or open a confirmation modal that lists them?
- Any platform/role scope to call out (web, extension, mobile)?
- Suggested UX copy for actions can be iterated with design.

This is similar to the banner received when CI checks are failing and may be done in microsoft/vscode-engineering.

[View original Slack conversation](https://vscodeteam.slack.com/archives/C09MVCZKLP4/p1786733317229989?thread_ts=1786733317.229989&cid=C09MVCZKLP4)