Replace hard validation error with self-healing token logic. Tokens that
are null, empty, or fail decryption are now regenerated automatically
rather than crashing sentinel startup or metrics reads.
Token format changed from encrypted JSON payload to a plain 64-char
random string (Str::random), eliminating double-encryption issues and
simplifying the validation regex to cover the new character set.
New `ensureValidSentinelToken()` method on ServerSetting centralises
the get-or-regenerate contract; both StartSentinel and HasMetrics now
delegate to it. HasMetrics logs a warning when regeneration occurs so
operators know a sentinel container restart is required.
`isValidSentinelToken()` now accepts `?string` (null → false).
Adds feature tests covering: null/empty/undecryptable stored values,
idempotent return of valid tokens, RuntimeException only when
regeneration itself produces an invalid token, no double-encryption of
newly generated tokens, and cast round-trip consistency.
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