fix(cli): classify errored provider purchase results as failures #1900

Problem

The CLI purchase path classifies a provider result using only PurchaseResult.Success.

If a provider returns PurchaseResult{Success: true, Error: nonNil}, purchaseSingleRec records audit status success and prints the success diagnostic even though the result contains a provider error. The MCP path had the same inconsistency and is being corrected in PR #1889.

Relevant current code:

• cmd/multi_service.go: purchaseSingleRec selects status and diagnostics from result.Success only.
• cmd/multi_service_helpers.go: executePurchase preserves a provider-returned result when the separate Go error is nil, including a non-nil result.Error.

Expected

CLI audit status and diagnostics should treat a purchase as successful only when result.Success is true and result.Error is nil. Returned provider data should remain compatible unless a separate API decision is made.

Acceptance criteria

• Add a regression using a provider result with Success=true and a non-nil Error.
• Require audit status error and a failure diagnostic for that result.
• Preserve existing behavior for ordinary success, Success=false, and a separate Go error.
• Verify the real purchaseSingleRec path, not only a helper predicate.

Found during adversarial review of PR #1889. This is intentionally separate because #1889 is scoped to MCP purchase auditing.