Sessions: fix stale CI checks when switching active session

[benibenj]

Problem

When switching between sessions, CI checks could display stale "running" status even though the PR had already been merged. Two root causes:

1. No immediate PR refresh on session switch — PR data was only updated on the next polling cycle (up to 30s for PR, 60s for CI), so switching sessions showed stale data.
2. CI lookups used branch name (headRef) — After a PR is merged and the head branch is deleted, GitHub's API returns 404 for the branch-based CI check lookup, leaving stale data in the observables.

Fix

Use commit SHA for CI lookups

• Added headSha field to IGitHubPullRequest (mapped from data.head.sha in the API response)
• Changed CI check lookup from pr.headRef (branch name) to pr.headSha (commit SHA), which remains valid even after branch deletion

Immediate refresh on session change

• Added GitHubActiveSessionRefreshContribution workbench contribution in the GitHub sessions contribution that watches the active session and immediately calls prModel.refresh() when the session changes
• Tracks _lastSessionResource to only refresh when the session actually changes

CI polling lifecycle

• Added ciModel.startPolling() / stopPolling() lifecycle management via reader.store in the CI model derived, ensuring polling is properly started for the active session and stopped when switching away

Files changed

• src/vs/sessions/contrib/github/common/types.ts — Added headSha to IGitHubPullRequest
• src/vs/sessions/contrib/github/browser/fetchers/githubPRFetcher.ts — Map head.sha from API response
• src/vs/sessions/contrib/github/browser/github.contribution.ts — New GitHubActiveSessionRefreshContribution
• src/vs/sessions/contrib/changes/browser/changesView.ts — Use headSha for CI lookups, manage CI polling lifecycle
• src/vs/sessions/contrib/github/test/browser/githubModels.test.ts — Updated test helper

[Copilot]

Pull request overview

Fixes stale GitHub PR/CI status in the Sessions window when switching the active session, especially for merged PRs where the head branch may be deleted.

Changes:

• Add headSha to the Sessions GitHub PR model and map it from the REST API response.
• Switch CI check lookups in the Changes view from headRef (branch) to headSha (commit SHA).
• Add a workbench contribution to immediately refresh PR data when the active session changes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/vs/sessions/contrib/github/common/types.ts	Adds headSha to IGitHubPullRequest so CI can be queried by commit.
src/vs/sessions/contrib/github/browser/fetchers/githubPRFetcher.ts	Maps data.head.sha into the PR model.
src/vs/sessions/contrib/github/browser/github.contribution.ts	Adds a contribution that refreshes PR data immediately on session change.
src/vs/sessions/contrib/changes/browser/changesView.ts	Uses headSha for CI lookups and starts/stops CI polling tied to the active session.
src/vs/sessions/contrib/github/test/browser/githubModels.test.ts	Updates test helper PR objects to include headSha.