[Bug type]
Behavior bug (incorrect output/state without crash)

[Summary]
PR #83172 fixes the Discord side of thread-bound subagent initial delivery by returning a routable deliveryOrigin from subagent_spawning after the channel binding succeeds. The same delivery-origin gap exists in the Feishu/Lark subagent hook: it binds the child session and returns threadBindingReady=true, but does not return the conversation/topic origin needed by the generic sessions_spawn path to deliver the initial child run directly to the bound target.

[Expected behavior]
When Feishu/Lark subagent_spawning successfully binds a child session to a DM or topic conversation, it should return a deliveryOrigin for that bound conversation so the initial child run can be delivered directly there.

[Actual behavior]
The hook returns only status=ok and threadBindingReady=true. The generic spawn path therefore cannot treat the bind as a routable child delivery target and may fall back to the requester route/completion behavior.

[Why this is complementary to #83172]
#83172 covers Discord. This issue tracks the adjacent Feishu/Lark hook behavior so channel thread-bound subagent sessions use the same contract across bundled providers.

[Scope]
Targeted Feishu/Lark hook change only: return the already-resolved bound conversation delivery origin from subagent_spawning and add regression coverage for DM/topic cases.

[Related]
Complements #83172 and #83170.