Gate agentSessionsWelcome view behind ExP service using cached treatment values

[Copilot]

The agentSessionsWelcome view needs to be controlled by the ExP service without blocking welcome page load on network calls.

Changes

• Added getCachedTreatment() to IWorkbenchAssignmentService: Returns synchronously from TAS client cache without awaiting network. Returns undefined if cache unavailable.

• Welcome page routing logic: Three entry points now check agentSessionsWelcome treatment via cached value:

  • AgentSessionsWelcomeRunnerContribution: Shows agent sessions welcome if treatment is true OR config explicitly set to agentSessionsWelcomePage
  • StartupPageRunnerContribution: Skips regular welcome when treatment is true and config is welcomePage
  • workbench.action.openWalkthrough: Redirects to agent sessions welcome when treatment is true

• Treatment constant: Extracted AGENT_SESSIONS_WELCOME_TREATMENT = 'agentSessionsWelcome' to shared location

Behavior

// Treatment enabled → shows agent sessions welcome
getCachedTreatment<boolean>('agentSessionsWelcome') === true

// Treatment disabled/unavailable → shows regular welcome (default)
getCachedTreatment<boolean>('agentSessionsWelcome') === false | undefined

// Override for local testing
"experiments.override.agentSessionsWelcome": true

Fallback to regular welcome preserves backward compatibility when cache is unavailable or treatment unset.

Original prompt

Can you make agentSessionsWelcome view to be behind the ExP service? Note: In order to determine whether to show the agent sessions welcome view or the regular welcome view we need to check the cache values of the exp service as opposed to delaying the welcome page load until the exp service is available

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