fix(fsapp): avoid premature task finalization

[Tomodad]

Summary

• keep the Feishu turn hook summary-only
• prevent turn-level exit_reason metadata from finalizing queued work early
• preserve display_queue's done item as the normal task-finalization path

Fixes #685.

Root cause

_make_task_hook received _finish as on_final and invoked it whenever a hook context contained exit_reason. That metadata can describe an intermediate agent turn rather than completion of the queued Feishu task, so _finish could set result["sent"] before the display queue published its real done item.

Change

Remove the final callback from _make_task_hook and let the hook handle only summary -> card.step. The existing display-queue done -> _finish path and timeout/stop/error failure paths are unchanged.

Verification

• isolated AST harness: 14/14 passed
• parent revision reproduces premature finalization
• candidate blocks exit_reason finalization while preserving summary updates
• adversarial cases cover missing response, empty summary, combined summary + exit_reason, repeated calls, inactive task IDs, and formatting inputs
• python -m py_compile frontends/fsapp.py
• git diff --check
• one file changed: 4 insertions, 8 deletions
• no dependencies added