test: improve critical workflow coverage

[nkavian]

Summary

• cover secure-storage backend failures, corrupt manifests, transactions, rollback, and file-permission enforcement
• cover MPP and x402 completion failures, timeouts, credential validation, and seller replay errors
• cover AEP-blocked MPP/x402 Inspect views and structured output
• make the AEP approval timeout test deterministic under full-suite concurrency

Coverage

• Core lines: 92.45% -> 95.08%
• Core branches: 85.76% -> 87.69%
• CLI lines: 91.14% -> 91.72%
• CLI branches: 80.36% -> 80.66%
• secure-storage lines: 94.35%
• shared payment-fetch helper: 100% statements, branches, functions, and lines
• MPP and x402 Inspect views: 100% lines

Verification

• pnpm format
• pnpm format:check
• pnpm typecheck
• pnpm lint
• pnpm test (Core 512/512; CLI 594 passed, 2 skipped)
• pnpm typedoc
• pnpm build
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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