Control-plane responses can now be redacted for hosted/background sessions

What

• New helper functions check whether a session redactsPersistedLane() (true for hosted/background sessions where the transport persists) and, when true, strip sensitive detail from SDK control responses.
• get_context_usage responses omit memoryFiles, and get_usage responses exclude usage behaviors data when the session's lane is redacted.
• MCP server config secrets are also stripped from responses in this case.

Why

This prevents potentially sensitive details, such as memory file contents, MCP config secrets, or usage behavior data, from being returned over the SDK control channel for hosted or background sessions where that data shouldn't be exposed.