[cherry-pick] sessions: gate Copilot CLI/Claude agent-host preferences on chat.agentHost.enabled

[vs-code-engineering]

Cherry-pick of #323567 from main.

What

Make chat.agentHost.enabled the master gate for two agent-host preference settings consumed by the Copilot Chat sessions provider in the Agents window:

• chat.agents.copilotCli.hideExtensionHost
• chat.agents.claude.preferAgentHost

When chat.agentHost.enabled is off, these two settings are no longer respected (treated as false).

Why

Both settings tell the Copilot Chat sessions provider to step aside so the agent host can surface its own entry in its place:

• hideExtensionHost hides the Extension Host Copilot CLI so only the Agent Host Copilot CLI shows.
• preferAgentHost (Claude) makes this provider yield so the agent host's Claude registration shows instead.

But when the agent host is disabled it never registers those replacement entries. Honoring the preferences in that state would make the Copilot CLI / Claude entries disappear entirely from the picker. Gating them on chat.agentHost.enabled keeps the entries visible until the agent host is actually enabled to take over.

Changes

• copilotChatSessionsProvider.ts: _isClaudeAvailable() and _isCopilotCliAvailable() now also check chat.agentHost.enabled (via a small shared _isAgentHostEnabled() helper). Availability is computed live from configuration; the cached fields were removed and the config-change handler simply re-collects/re-sorts session types when any relevant setting changes.
• agentHost.config.contribution.ts / chat.shared.contribution.ts: updated the two setting descriptions to note they require #chat.agentHost.enabled#.
• Added unit tests covering that each preference is ignored while the agent host is disabled, and that toggling chat.agentHost.enabled live-applies the gate.

Notes for reviewers

• This is a read-time override: the user's stored values are untouched, just ignored while the master is off.
• chat.agentHost.defaultSessionsProvider was already gated on chat.agentHost.enabled (the local agent host provider only registers when enabled), so no change was needed there.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

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