fix(payment): isolate provider mempool errors

[Alike001]

Summary

• isolate any per-transaction provider error after a successful mempool snapshot
• continue scanning when other mempool candidates decode successfully
• pause safely when every candidate fails or an incomplete scan would expire an invoice
• add regression tests for isolated provider limits and total candidate failure

Production evidence

The live invoice completed getblockcount, getaddresstxids, getrawmempool, and 15 getrawtransaction calls before one candidate failed. The endpoint and invoice were both healthy; the isolated candidate was misclassified as a total outage.

Verification

• npm run format:check
• npm run lint
• npm run typecheck
• focused payment suite: 21 passed
• remaining suite: 133 passed, 1 skipped
• npm run secret:check
• npm run build

Closes #35

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Project	Deployment	Actions	Updated (UTC)
zecceipt	Ready	Preview	Aug 30, 2026 1:54pm