Add `clientByokEnabled` to `IChatEntitlementService` (#308553)

Add clientByokEnabled to IChatEntitlementService

Adds a clientByokEnabled boolean to IChatEntitlementService that reads
the github.copilot.clientByokEnabled context key set by the Copilot Chat
extension. This enables the Add Models button for Business/Enterprise
users whose organization has the BYOK policy enabled.

- Gate with isManagedEntitlement for defense-in-depth
- Listen to onDidChangeContext for reactive button updates

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