Scope legacy tool-only prompt rules when adopting managed voice delegation

Scope legacy tool-only prompt rules when adopting managed voice delegation. A successful function result can still yield a silent conversation if the reused routing prompt says to output no text and the managed backend obeys that rule on its post-tool continuation. State explicitly that the no-text rule applies only while selecting a tool for a new caller turn; after function_call_output, return the approved reply as nonempty final text and finish without another tool call. Keep the business tool permissions and confirmations enforced by application handlers.

Applicability and limits: This applies to systems whose old transport spoke handler output itself but whose new managed backend must return text to a native voice model. It does not establish causes of unrelated audio, transcription or delegation latency. A connection handshake or successful handler invocation alone does not test the post-tool reply.