Fix reached_preemption crash when JAX preemption service is disabled

[marsunique]

Description

Fixes #3406

When JAX's preemption service is disabled via jax_enable_preemption_service=False,
multihost_utils.reached_preemption_sync_point raises a RuntimeError because the
preemption sync manager is None. This causes CheckpointManager.save() to crash.

This change catches the RuntimeError in reached_preemption() and returns False,
gracefully treating a disabled preemption service as "no preemption detected".

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (alters existing behavior or a public API)
• Docs / tests / internal tooling (no user-facing behavior change)

Checklist

• I have read the contribution guidelines.
• Tests cover this change and pass locally.
• Public APIs and non-trivial functions are documented.
• If this change is user-facing, I have updated CHANGELOG.md under the [Unreleased] section (or the relevant checkpoint/ / export/ changelog).