zaps: stop spurious "failed to handle zap" warnings on non-zap invoices

The zap dispatch parsed the invoice description and, when it wasn't JSON,
suppressed the error only if the message contained "Unexpected" — V8's parse-
error wording. On Bun the message is "Unable to parse JSON string", so every
paid invoice with a non-JSON memo logged a warning. Parse-then-check instead:
a non-JSON description is simply not a zap and is skipped silently; warn only
when an actual kind-9734 zap request fails to process.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

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