Cloud agents: preserve archive state across v1↔v2 backend flip

[osortega]

Why

When the cloud-agent backend flips from Jobs API (v1) to Task API (v2), the same PR-backed session would change URI shape from /<prNumber> to /task/<id>. The VS Code chat sessions host stores archive (and pin) state keyed by resource URI, so a flip silently un-archives every previously archived item — surfacing dozens of stale closed/merged PR sessions in users' sidebars.

This change keeps the URI stable across the flip.

What

• Always emit /<prNumber> URIs when a PR is resolvable, regardless of which backend produced the entry.
• PR-less tasks (Task API) keep their /task/<id> URIs — there's no PR number to migrate to.
• Newly-created tasks still start as /task/<id> (the PR doesn't exist at creation time); the first list refresh after the PR materializes upgrades the URI to /<prNumber> for subsequent renders.

To keep this safe, PR-shaped URIs on the v2 backend now reverse-resolve to the underlying task before dispatching:

• provideChatSessionContent → routes to provideTaskChatSessionContent(taskId).
• openSessionInBrowser → uses task.html_url.
• handleFollowUp → routes to handleTaskFollowUp(taskId).

Reverse resolution hits a per-provider _taskIdByPrNumber cache populated at list time, falling back to a targeted listTasksForRepo({ artifact_type: 'pull', artifact_id }) lookup on cold URIs.

Files

• extensions/copilot/src/extension/chatSessions/vscode/cloudAgentBackend.ts — new findTaskIdForPullRequest on TaskCloudAgentBackend.
• extensions/copilot/src/extension/chatSessions/vscode-node/taskApiBackend.ts — implementation via existing listTasksForRepo.
• extensions/copilot/src/extension/chatSessions/vscode-node/copilotCloudSessionsProvider.ts — cache, helper, URI emission change, and three consumer-site dispatches.

Out of scope (follow-up)

A durable fix that decouples archive state from URI shape entirely (provider-managed archive store keyed by stable id, layered on the createChatSessionItemController migration) is planned as a follow-up. This PR is the bridge that prevents user-visible regression while that lands.

Verification

• Compile clean across the three modified files.
• No host-side changes.

[Copilot]

Pull request overview

This PR updates the Copilot Cloud Sessions provider to keep PR-backed session URIs stable (/<prNumber>) across the Jobs API (v1) ↔ Task API (v2) backend flip, so archived/pinned state (keyed by URI) is preserved even when the underlying backend representation changes.

Changes:

• Adds a Task-backend reverse-lookup API (findTaskIdForPullRequest) to map PR numbers back to task IDs on v2.
• Updates the sessions provider to always emit PR-number URIs when a PR is resolvable, and to reverse-resolve PR URIs to tasks for content rendering, browser opening, and follow-ups on v2.
• Introduces an in-memory PR→taskId cache populated during listing to avoid extra network lookups.

Show a summary per file

File	Description
extensions/copilot/src/extension/chatSessions/vscode/cloudAgentBackend.ts	Extends the Task backend interface with a PR→task reverse-lookup method.
extensions/copilot/src/extension/chatSessions/vscode-node/taskApiBackend.ts	Implements PR→task lookup using Task API list filtering.
extensions/copilot/src/extension/chatSessions/vscode-node/copilotCloudSessionsProvider.ts	Emits stable PR URIs on v2 when PRs resolve; adds PR→task cache and v2 routing for content/open/follow-up.

Copilot's findings

• Files reviewed: 3/3 changed files
• Comments generated: 5