Manually exit the active detour and return to the parent practice.

Mirrors the auto-exit flow in the main practice endpoint:

• Pop detour_stack snapshot
• Restore parent current_question / completed / skipped / observations
• Set last_completed_detour so the NEXT turn's system prompt can inject the welcome-back augmentation
• Clear active_detour_practice_id and pending_detour

Resolution order (first match wins):

1. body.session_id → update that row directly (auth-independent)
2. current_user's student → their in-progress session for this module
3. Single incomplete session matching module_type (dev fallback)

Idempotent — returns {"exited": false} if there's no active detour.