Skip chat extension enablement change when locked by environment (#315453)

When chat.disableAIFeatures is true and the chat extension is in a non-changeable enablement state (DisabledByExtensionKind, DisabledByVirtualWorkspace, DisabledByAllowlist, etc.), ChatTeardownContribution.maybeEnableOrDisableExtension calls setEnablement which throws an unhandled error. Guard with canChangeEnablement / canChangeWorkspaceEnablement before calling.

Fixes #312381