Drop `propagate_traces`

[sentrivana]

Drop the deprecated propagate_traces init option in 3.0.

Important: The Celery integration also has an option called propagate_traces -- this should remain unchanged.

POTel PR: #4206

[linear]

PY-1944

[IgorGanapolsky]

Agentic systems are turning into always-on factories: one request fans out into model calls, tool calls, policy checks, and storage.

The hard part is rarely tokens/sec — it is control-plane reliability:

1. Fan-out control — hard cap on tools/steps per request before external writes
2. Durable id + idempotency — one write per business object (ledger / preflight)
3. Observable long jobs — progress + cooperative cancel on multi-minute runs
4. Expected vs actual — durable artifact id before marking success
5. Policy before action — gate that can refuse known-bad side effects

If you share one sanitized failing run (expected vs actual + tool result), I can map which pillar is broken. Architecture only.