feat(purchase-order): cover purchase_order_number payloads

[tiagolupepic]

Add purchase order number support

• Subscription
• Wallets and Wallet Recurring rules
• Wallet Transactions

[ancorcruz]

Direction looks right, and the mock_fetch removal is worth merging on its own — deno task test is currently broken on main.

Three things before merge, details inline:

1. The satisfies X & { purchase_order_number: string } intersections aren't needed — the current spec already types purchase_order_number?: string | null on every payload touched here — and they make the fixtures unable to fail, which defeats the point of adding them.
2. tests/wallet.test.ts:61-62 adds two new type errors (credits are string in WalletRecurringTransactionRule). Measured against a freshly generated client: 77 errors on main → 79 here.
3. Worth being explicit that none of this runs in CI: .github/workflows/ci.yml runs deno task build + deno task typecheck, and typecheck only covers mod.ts, webhook_types.ts and tests/webhook_types.test.ts. deno task test isn't wired in, and the suite fails type-check with 77 pre-existing errors (stale names WalletInput → WalletCreateInput, Wallets → WalletsPaginated, Subscriptions → SubscriptionsPaginated, plus number-vs-string amounts). So as merged these assertions are documentation, not a safety net. This PR clears the biggest blocker — a follow-up that fixes the fixture drift and adds deno task test to CI is where the value actually lands.

Two coverage gaps vs the description, if you want them in scope:

• WalletUpdateInput.wallet.recurring_transaction_rules[].purchase_order_number exists in the spec but isn't covered (only the top-level field is in walletUpdateInput).
• Nothing exercises purchase_order_number: null, which the spec allows.