fix(decisioning): disable sync task webhooks by default

[bokelley]

Summary

• default synchronous terminal webhook auto-emission to off for AdCP conformance
• preserve explicit legacy compatibility while keeping real TaskHandoff delivery independently enabled
• reject undeliverable push handoffs before task creation and release proposal reservations
• update migration guidance, examples, and regression coverage

Closes #1019.
Supports adcontextprotocol/adcp#5389.

Validation

• full suite: 6259 passed, 41 skipped, 9 deselected, 1 xfailed
• Ruff, Black, mypy, Bandit, and repository pre-commit hooks passed
• independent code, protocol, and security reviews found no merge blockers

[aao-ipr-bot]

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