chore: sync listCharges fixture with certificate-purchase entry

[ggalmazor]

Syncs the listCharges fixture with upstream dnsimple/dnsimple-developer#986.

Adds a certificate purchase entry to demonstrate that product_reference is a string even when the reference is a numeric ID.

Note: this client does not yet implement a Billing service, so there are no tests to update — only the fixture is synced.

Checklist

• Close and re-open this PR to trigger the CI workflow (it does not run automatically)
• Ensure the CI workflow passes before merging

[ggalmazor]

No test changes in this PR because the Rust client does not currently implement a Billing service. A quick audit confirms:

• src/dnsimple/ has no billing.rs module and no Charge / ChargeItem structs.
• tests/ has no billing or charges test file.
• grep -ri "billing\|charge" src/ tests/ returns no matches outside the fixture itself.

The listCharges/success.http fixture exists as a stub for a future Billing implementation, but no code consumes it today. Because there's no consumer, there's no failing assertion to drive via TDD — syncing the fixture is the only applicable change here.

Once a Billing service lands in this client, the sync'd fixture will be ready to back its tests (including the certificate-purchase case that motivates the upstream fixture change).