Agents - refactor GitHub models

[lszomoru]

• Use ReferenceCollections for each model kind
• Centralize refresh and polling into one workbench contribution

[github-actions]

blocks-ci screenshots changed

Replace the contents of test/componentFixtures/blocks-ci-screenshots.md with:

Updated blocks-ci-screenshots.md

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#### editor/codeEditor/CodeEditor/Dark
![screenshot](https://hediet-screenshots.azurewebsites.net/images/cb32a3e854b5734fe5aaca2318f2e0a42ee821b05ea97883ea42c5ba95edb3c3)

#### editor/codeEditor/CodeEditor/Light
![screenshot](https://hediet-screenshots.azurewebsites.net/images/42624fbba5e0db7f32c224b5eb9c5dd3b08245697ae2e7d2a88be0d7c287129b)

#### editor/inlineChatZoneWidget/InlineChatZoneWidget/Dark
![screenshot](https://hediet-screenshots.azurewebsites.net/images/28af1b719f3877d2f048fef10bb4534d5f638c1987bb0536331f4ab50ea0ea7a)

#### editor/inlineChatZoneWidget/InlineChatZoneWidget/Light
![screenshot](https://hediet-screenshots.azurewebsites.net/images/c6c77ca2785665ae66c817cda21a8e5f5ea7d51e798c5b4e5aedcda9fd7323bd)

#### editor/inlineChatZoneWidget/InlineChatZoneWidgetTerminated/Dark
![screenshot](https://hediet-screenshots.azurewebsites.net/images/2fbc12507b59ff950d9612d2df92e6b39d8bf0bf500478e42eca2ead4d1ae206)

#### editor/inlineChatZoneWidget/InlineChatZoneWidgetTerminated/Light
![screenshot](https://hediet-screenshots.azurewebsites.net/images/4632ab04d1fdd7db9ab0e00cce10aefb7a6344eb8869dfce740309a8801cab73)

[Copilot]

Pull request overview

This PR refactors the sessions-window GitHub integration to move model ownership from ad-hoc caches to reference-counted collections, and to centralize active-session refresh/polling logic in a single workbench contribution. It fits into the vs/sessions layer by consolidating how PR, CI, and review-thread data are shared across the Agents experience.

Changes:

• Replaced direct GitHub model getters with reference-returning APIs backed by per-model ReferenceCollections.
• Added active-session observables in GitHubService and moved CI/review-thread refresh + polling into GitHubPullRequestPollingContribution.
• Updated dependent services/tests to consume model references or active-session observables instead of direct cached models.

Show a summary per file

File	Description
src/vs/sessions/contrib/github/test/browser/githubService.test.ts	Updates service tests from direct cached models to reference-based acquisition/disposal behavior.
src/vs/sessions/contrib/github/test/browser/githubModels.test.ts	Adjusts CI model tests to exercise reference collection behavior.
src/vs/sessions/contrib/github/test/browser/githubContribution.test.ts	Updates polling contribution tests/mocks for the new service API.
src/vs/sessions/contrib/github/browser/models/githubRepositoryModel.ts	Adds a reference collection wrapper for repository models.
src/vs/sessions/contrib/github/browser/models/githubPullRequestReviewThreadsModel.ts	Adds a reference collection wrapper for PR review-thread models.
src/vs/sessions/contrib/github/browser/models/githubPullRequestModel.ts	Adds a reference collection wrapper and changes PR polling bookkeeping to disposable-based tracking.
src/vs/sessions/contrib/github/browser/models/githubPullRequestCIModel.ts	Adds a reference collection wrapper and includes PR number in CI model state/logging.
src/vs/sessions/contrib/github/browser/githubService.ts	Reworks the GitHub service around references plus active-session derived observables.
src/vs/sessions/contrib/github/browser/github.contribution.ts	Centralizes active-session PR/CI/review-thread refresh and polling behavior.
src/vs/sessions/contrib/copilotChatSessions/browser/copilotChatSessionsProvider.ts	Switches live PR icon computation to use a model reference.
src/vs/sessions/contrib/codeReview/test/browser/codeReviewService.test.ts	Updates code review tests for the new active-session review-thread flow.
src/vs/sessions/contrib/codeReview/browser/codeReviewService.ts	Refactors PR review state consumption to read from the GitHub service’s active-session review-thread observable.
src/vs/sessions/contrib/changes/browser/checksViewModel.ts	Switches checks view model to consume the active-session CI observable.
src/vs/sessions/contrib/changes/browser/checksActions.ts	Switches failed-check actions/context to consume the active-session CI observable.

Copilot's findings

• Files reviewed: 14/14 changed files
• Comments generated: 4