Fix prefix health checks after config reloads

[tionis]

This should fix prefix health-check handling during central config reloads.

Previously, Nylon reconciled advertised prefixes by prefix address only. If a config reload kept the same prefix but changed its health-check configuration, the prefix was treated as already active. Nylon then rewired the advertisement to the newly unmarshaled health-check object without starting it. For HTTP/ping health checks, that could leave the advertised metric at the object’s zero/default value instead of reflecting probe health.

Now, equivalent prefix health configs keep the existing running checker. Changed prefix health configs stop the old checker, start the new checker, and wire the advertisement to the running checker.

This matters for health-gated anycast prefixes distributed through central config updates. Without this fix, a provider can continue advertising a prefix as healthy after a reload even though the new health checker was never started.

Tests

• Added regression coverage for changing an existing prefix to HTTP health.
• Added regression coverage for equivalent reloads preserving the running health checker.
• Ran go test ./core
• Ran go test ./...
• I did not run e2ee tests as I don't have a non-root docker setup on my machine.

[Copilot]

Pull request overview

This PR fixes prefix health-check behavior across central config reloads by distinguishing “same prefix, same health config” from “same prefix, changed health config,” ensuring health check goroutines are started/stopped correctly and advertisements point at running checkers.

Changes:

• Reworks reconcileAdvertisedPrefixes to reuse existing running prefix health checkers when the effective health-check configuration is unchanged, and to stop/start checkers when it changes.
• Adds samePrefixHealthConfig (plus helpers) to compare health-check configs while accounting for tunable defaults.
• Adds regression tests covering changed health-check configs starting correctly and unchanged configs reusing the running checker.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
core/nylon_apply.go	Updates advertised-prefix reconciliation to preserve or restart prefix health checkers based on health config equivalence, and adds config-comparison helpers.
core/nylon_apply_test.go	Adds tests validating correct start/reuse behavior of prefix health checkers during config reload reconciliation.

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[encodeous]

Thanks for this PR!