fix(purchase): surface save error on failed-status execution record

[cristim]

Problem

COR-10 (#1184): in executeForAccount (internal/purchase/execution.go), the credential-resolution failure branch discarded the SavePurchaseExecution error (_ =), while the post-purchase save thirty lines later treats a save failure as AUDIT LOSS and returns it. If the DB write failed on the credential-failure path, the per-account "failed" row was silently lost with no log and no error, leaving a gap in the History audit trail.

Fix

Handle the failure-path save exactly like the success path: when the save fails, return an AUDIT LOSS: failed to save execution record ... error joined (via errors.Join) with the original credential-resolution error, so neither failure is masked and the underlying save error stays inspectable via errors.Is.

Test evidence

• New regression test TestExecuteForAccount_CredentialFailure_SaveErrorSurfaced replicates the real scenario (credential resolution fails AND the failed-row save fails) and asserts both the AUDIT LOSS error and the credential error surface.
• Confirmed it FAILS on pre-fix code (save error silently dropped) and passes with the fix.
• go build ./... clean; go test ./internal/purchase/... -count=1: 207 passed; go vet clean.
• Per the finding guidance, execution_test.go timing assertions were not touched (TEST-05 / TEST-05: Elapsed-wall-time parallelism assertion in purchase fan-out test is CI-load sensitive #1158 is separate).

Closes #1184

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📥 Commits

Reviewing files that changed from the base of the PR and between 4108e51 and 8a08afd.

📒 Files selected for processing (2)

• internal/purchase/execution.go
• internal/purchase/execution_test.go

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• Create PR with unit tests
• Commit unit tests in branch fix/cor-10-fix

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[cristim]

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