Headless On-Ramp Checkout API in mobile

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**What is this about?**

**Goal**
Enable a **headless buy flow** so external features (like MM Pay confirmation) can launch on-ramp checkout directly, without showing `TokenSelection` / `BuildQuote` (this is critical as we already know what we need to buy and the amount) screens, and receive `orderId` programmatically.

**Why We Need This**
- We need to integrate buy flow into confirmation UX.
- Current integration requires stitching together internal screens/routing (`BuildQuote`, native OTP flow, checkout callbacks), which is brittle.
- Callback handling differs between provider flows (native vs aggregator), and this leaks complexity to feature teams.
- We need `orderId` immediately to store in `TransactionPayController` and later orchestrate post-fiat steps (Relay).

**Requested Capability**
1. Expose a public, supported headless API from the Ramps module/hook (not screen-coupled).
2. Allow caller to start buy with preselected context:
- `assetId`
- `amount`
- `currency` (we currently need forced `USD`)
- `providerId`
- `paymentMethodId`
- `walletAddress`
- `metadata` // Optional object: This needs to be included into `order` at the end of checkout, so that we can send `{ transactionId: "XXX"} ` and can do regular checks on `RampsController` state to understand if order is filled
3. Ramps handles provider-specific auth/checkout/callback internally.
4. Ramps returns lifecycle events to caller:
- `onOrderCreated(orderId, providerId)`
- `onOrderStatusChange(orderId, status)`
- `onCheckoutClosed(reason)`
- `onError(error)`
5. Preserve ability to render checkout WebView/modal, but caller should not manage intermediate Ramps pages.
6. Ensure order creation callback works for both native and aggregator providers consistently.

**Expected API Shape (Example)**
```ts
startHeadlessBuy({
assetId,
amount,
currency: 'USD',
metadata,
providerId,
paymentMethodId,
walletAddress,
onOrderCreated,
onOrderStatusChange,
onClose,
onError,
});
```

**Acceptance Criteria**
- No navigation to BuildQuote in headless mode.
- Works for native and aggregator providers.
- `orderId` is reliably emitted when available.
- Caller can close/dismiss and return to its own screen without custom route hacks.
- Callback URL differences are fully abstracted by Ramps.
- Public docs/example added for integration teams.

**Non-goals (For This Task)**
- UI redesign of existing Ramps screens.
- Reworking non-headless buy flow behavior.

**Demo recording after feature implemented**
https://github.com/user-attachments/assets/c1735559-32a4-40f1-9c5a-dc3e7b31776a

**Stakeholder review needed before the work gets merged**

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

Start by locating the Ramps module or hook and tracing the existing TokenSelection, BuildQuote, native, and aggregator checkout paths. Review how TransactionPayController and RampsController currently receive order data and status. Done means a documented headless buy API handles both provider types, emits the specified lifecycle events, avoids BuildQuote navigation, and supports caller dismissal.

Written by the indexing model from the issue text.