feat: support one-off charges via mode: oneOff

[BenKalsky]

Summary

Adds one-off charge support to createSumitChargeRoute via a new mode option, paired with the new builder/alias landing in sumit-api#11 (`buildOneOffChargePayload` + `normalizeChargeResponse`).

What's new

```ts
createSumitChargeRoute({
companyId,
apiKey,
mode: "oneOff", // default "recurring" — back-compat
onResult,
});
```

`mode`	Endpoint	Required item fields
`"recurring"` (default)	`POST /billing/recurring/charge/`	name, description, unitPrice, currency, durationMonths
`"oneOff"`	`POST /billing/payments/charge/`	name, description, unitPrice, currency

The same `` and `SingleUseToken` work for both — only the route's `mode` changes. The validator no longer requires `durationMonths` in one-off mode but still requires it in recurring mode.

Internally, response normalization switched from `normalizeRecurringChargeResponse` to its new alias `normalizeChargeResponse` (same function, clearer name).

Test plan

• `pnpm test` — 31/31 passing (3 new tests: oneOff happy path, relaxed validation, recurring still requires durationMonths)
• `pnpm typecheck` clean
• `pnpm build` clean

Depends on

sumit-api#11 — must merge first since this PR imports the new `buildOneOffChargePayload` and `normalizeChargeResponse`.

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[chatgpt-codex-connector]

Bump required sumit-api version for new imports

This change adds static imports for buildOneOffChargePayload and normalizeChargeResponse, but package.json still allows sumit-api >=0.1.0; any install that resolves an older compatible version will fail module loading before the handler runs (including recurring-only usage) because ESM imports are resolved eagerly. Please raise the peer/dev dependency minimum to the first sumit-api release that exports these symbols so consumers do not hit runtime/boot-time import errors.

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