[Refactor] Extract Callback Activation Logic

[yedidyakfir]

Description

Extract callback activation from HatchetInvoker to a reusable, idempotent module.

Blocked by: #40, #42

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Current Location

• /mageflow/invokers/hatchet.py - run_success(), run_error()
• /mageflow/signature/model.py - activate_success(), activate_error()

Target

• Create /mageflow/orchestration/callbacks.py with idempotent activation
• Add callbacks_activated flag to TaskSignature
• Both in-task and external service use same logic