exp: set exp header if snippy is in blocking mode

[ulugbekna]

No description provided.

[Copilot]

Pull request overview

Adds support for an sn (Snippy blocking mode) flag from the Copilot token into the experimentation user info cache and ExP filter headers, enabling experiment targeting based on Snippy’s blocking state.

Changes:

• Parse sn=1 from the Copilot token (CopilotToken.isSn()).
• Persist isSn in UserInfoStore and include it in GitHub account ExP filter headers (X-GitHub-Copilot-IsSn).
• Add unit tests validating isSn detection and caching behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/platform/telemetry/vscode-node/microsoftExperimentationService.ts	Adds X-GitHub-Copilot-IsSn filter and logs isSn alongside other user filters.
src/platform/telemetry/test/node/experimentation.spec.ts	Adds tests for sn flag detection and persistence via UserInfoStore/globalState.
src/platform/telemetry/node/baseExperimentationService.ts	Extends UserInfoStore to track/persist isSn and treat it as part of change detection.
src/platform/authentication/common/copilotToken.ts	Adds isSn() token flag accessor based on sn=1.