fix(exchange): fail closed on unparseable PaymentDue in daily-cap check

[cristim]

Problem

COR-04 (docs/reviews/codebase-review-2026-06-10.md): in processAutoExchange (pkg/exchange/auto.go), a PaymentDue parse failure only logged a warning and coerced the value to $0 before adding it to the MaxPaymentDailyUSD daily-cap check. This is a silent fallback on a money path and is inconsistent with the dailySpent parse failure five lines above, which aborts the exchange and persists a failed record. Latent today (current callers always produce a parseable decimal), but any future path producing a non-decimal paymentDueStr would silently undercount the daily guardrail.

Fix

• The PaymentDue parse failure now mirrors the dailySpent branch: log at error level, set outcome.Error, persist a failed record via saveFailedRecord, and return without executing the exchange.
• The documented nil-means-zero-cost quote case stays explicit and separate: processRecommendation maps a nil quote.PaymentDueUSD to the literal "0" string before the parse, now with a comment distinguishing it from parse errors.

Test evidence

New regression test TestProcessAutoExchange_UnparseablePaymentDue_FailsClosed exercises processAutoExchange directly with "not-a-number" and "" (the public RunAutoExchange path always builds a parseable string, which is why the bug was latent). It asserts the outcome errors, Execute is never called (new mock call counter), and a failed record is persisted.

• Pre-fix (fix stashed): both subtests FAIL - outcome.Error empty, exchange executed with $0 counted toward the cap.
• Post-fix: go test ./exchange/ from pkg/ - 97 passed.
• go build ./... succeeds in both the root module and the pkg/ module.

Closes #1166

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📥 Commits

Reviewing files that changed from the base of the PR and between 89658a3 and 0099c1a.

📒 Files selected for processing (2)

• pkg/exchange/auto.go
• pkg/exchange/auto_test.go

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[cristim]

Rebased onto main (includes #1437 tflint cache + #1438 trivy skip). The previous Security Scanning CI failure was on the pre-rebase commit; gosec v2.27.1 now passes on all 6 modules (root, pkg, providers/{aws,azure,gcp}, tests/e2e) with 0 issues. Gate results: go build/vet exit 0 (root + pkg), gocyclo on pkg/exchange exit 0, go test ./exchange/... 103 passed, gosec all modules exit 0.