fix(mpp): require receipt method and timestamp

[cryptoryda]

Require method and timestamp in Payment receipts

Summary

Reject Payment-Receipt headers that omit the required method or timestamp fields.

Previously, ParsePaymentReceipt accepted these malformed receipts and returned an empty method or a zero-value time.Time. This makes receipt validation stricter and prevents incomplete receipts from being treated as valid.

Changes

• Require a non-empty timestamp before parsing it.
• Require a non-empty method before validating its syntax.
• Preserve the existing timestamp-format and method-name validation.
• Add regression coverage for missing and empty values for both fields.

Testing

• go test ./pkg/mpp
• go vet ./...
• go test -race -count=1 ./...