fix(healthcheck): guard periodic checks against divide-by-zero when config is cleared

[mvanhorn]

Description

The healthcheck goroutine can panic with panic: runtime error: integer divide by zero (stack at internal/healthcheck/checker.go in fullPeriodicCheck).

fullPeriodicCheck indexed the shared config slice directly with c.tlsDialAddrs[try%len(c.tlsDialAddrs)]. Checker.Stop() clears the config by setting c.tlsDialAddrs = nil (and icmpTargetIPs / smallCheckType) — but did so without holding c.configMutex. A fullPeriodicCheck still in flight during or just after Stop() then evaluates try % len(c.tlsDialAddrs) with len == 0 and panics. It is also a data race: fullPeriodicCheck read c.tlsDialAddrs without the mutex while Stop/SetConfig wrote it. smallPeriodicCheck already copies c.icmpTargetIPs under the mutex, but its icmpTargetIPs[try%len(icmpTargetIPs)] had the same latent divide-by-zero when that copy is empty (Stop nils icmpTargetIPs too).

This change:

• Stop() now takes c.configMutex while clearing the config, synchronizing the clear with the readers.
• fullPeriodicCheck() copies c.tlsDialAddrs into a local slice under c.configMutex (mirroring smallPeriodicCheck) and returns early when the copy is empty — there is nothing to dial — instead of dividing by zero. The check closure indexes the local copy.
• smallPeriodicCheck() returns early when the (already-copied) ICMP target slice is empty and the check type is not DNS, so the copied-slice modulo cannot divide by zero either. The DNS-fallback path is unchanged.

Behavior is identical for a normally-running checker with non-empty config; the guards only affect the teardown / empty-config window.

Verified with go test ./internal/healthcheck/..., go test -race ./internal/healthcheck/..., go build ./..., and gofmt -l (clean). A regression test exercises the periodic checks with empty target slices and asserts they no longer panic.

Issue

Closes #3398

Assertions

• I am aware that any changes to settings should be reflected in the wiki — N/A, this change adds no settings and does not alter existing settings.