[jeremydmiller]

Backport of main's a0772b571 (#3522) to the 5.x maintenance line. Found while auditing what OTel work main had that 5.x didn't during #4117; this was the remaining gap.

The bug

In DurabilityMode.Solo, StartSoloModeAsync scheduled the recurring health-check loop with a fire-and-forget Task.Run inside the startup wolverine_node_assignments activity's using block. Task.Run captures the ambient ExecutionContext — including the AsyncLocal behind Activity.Current — so every loop iteration, and every DB call underneath it, reparented itself to that one stopped-but-still-ambient activity for the entire process lifetime. The result is a single OTel trace that grows without bound for as long as the host runs.

Because ShouldTraceHealthCheck() was also only evaluated once at startup, NodeAssignmentHealthCheckTraceSamplingPeriod was a no-op in Solo mode.

The fix

Three parts, identical to main:

1. Close the startup activity's scope before the recurring loop is scheduled.
2. Fork the loop inside ExecutionContext.SuppressFlow() so Activity.Current is never captured into it.
3. Start a fresh, bounded activity per tick, gated by ShouldTraceHealthCheck() — so each tick is its own root trace and the sampling period is finally honored.

Deliberately out of scope

Main's copy of startAllAgentsAsync also carries the GH-3519 per-agent start isolation (each agent's start wrapped in its own try/catch so one wedged agent doesn't skip its healthy siblings). That is a separate behavior change from a different commit and is not included here — this PR is GH-3518 only. Happy to do it as its own backport if wanted.

Testing

• New solo_mode_health_check_tracing (2 tests, ported from main). Red baseline verified: with the test in place and the source fix reverted, each_recurring_tick_is_its_own_bounded_root_trace fails on the per-tick activity count, and passes again once the fix is restored.
• Worth being precise about the second test: sampling_period_throttles_solo_mode_recurring_checks passes vacuously against the unfixed code (no per-tick activities exist to throttle). It is meaningful only paired with the first test, which establishes that ticks trace at all — same as on main.
• Full CoreTests: 1691 passed, 0 failed on net9.0.
• Full wolverine.slnx builds clean pinned to net9.0 (Release).

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