[agentserver] Gate blocking boot-time task recovery scan on the enablement flag

[Nathandrake229]

Summary

The resilient TaskManager's cold-start recovery scan (Layer 1) blocks server lifespan startup on a task-store list() + credential-token acquisition. It previously ran whenever either a durable task was registered or the enablement switch was set.

Because the responses protocol registers internal @task / @multi_turn_task primitives at host construction, _has_registered_tasks() was always True — so every responses host, even a plain one that opted into no resilient features, paid the blocking boot-time scan.

This PR makes the blocking Layer 1 scan opt-in via the existing set_resilient_tasks_enabled() switch, while keeping background (Layer 2) and request-time (Layer 3) recovery always available.

Changes

Core

• TaskManager.startup() gains run_initial_scan (default True — preserves the contract for direct callers and the recovery test-suite). When False, the blocking cold-start scan is skipped; the periodic background loop (Layer 2) and request-time inline reclaim (Layer 3) still provide recovery.
• The hosted lifespan (_base.py) passes run_initial_scan=_resilient_tasks_enabled() — a host that did not enable resilient tasks boots fast, keeping Layers 2 and 3 available.

Samples

• The invocations resilient samples (resilient_research, resilient_multiturn, resilient_langgraph) declare developer @multi_turn_tasks and now call set_resilient_tasks_enabled(True) to retain eager boot recovery — matching the responses resilient samples (19–22), which already do this.

Behavior

Deployment	Layer 1 (boot scan)	Layers 2 + 3
Switch ON (set_resilient_tasks_enabled(True))	runs at boot (unchanged)	available
Switch OFF (plain responses host, or @task app that didn't opt in)	skipped (was: ran)	available

Trade-off: for switch-off hosts, proactive reclaim of orphans from a prior crash moves from boot-time to the background loop (~interval) — anything re-requested is still reclaimed immediately by Layer 3. Ideal practice is to enable the switch; this change makes the un-enabled path degrade gracefully instead of blocking boot.

Backward compatibility

• No public API break: startup() only gains an optional keyword defaulting to today's behavior; it is internal (called by _base.py).
• No decorator / TaskOptions changes; no version-pin bump (set_resilient_tasks_enabled already shipped).

Tests

• test_task_manager_optin.py updated: fake startup accepts the kwarg; added run_initial_scan assertions (skipped when switch off, runs when on). 11 passed.
• Core recovery suite unaffected (default run_initial_scan=True): test_inline_recovery / test_recovery_lease_etag / test_lifecycle — 28 passed.

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