feat(ramps): add providers state and getProviders method to RampsController (#7652)

## Explanation

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## Add providers state and getProviders method to RampsController

Adds `providers` state to RampsController and fetches providers from the
V2 API endpoint.

**Changes:**
- Added `providers: Provider[]` to `RampsControllerState`
- Added `getProviders()` method to `RampsService` that calls
`/v2/regions/:regionCode/providers` with optional filters (provider,
crypto, fiat, payments)
- Added `getProviders()` method to `RampsController` with caching and
state management
- Providers are automatically fetched on `init()` when a region is
available
- Providers are automatically refetched when the region changes in
`updateUserRegion()` and `setUserRegion()`
- Providers are cleared when the region is cleared or changed

**Testing:**
- Added comprehensive tests for `getProviders` functionality
- Updated existing tests to include providers in state snapshots
- All 74 tests passing

## References

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https://consensyssoftware.atlassian.net/browse/TRAM-3153


## Checklist

- [ ] I've updated the test suite for new or updated code as appropriate
- [ ] I've updated documentation (JSDoc, Markdown, etc.) for new or
updated code as appropriate
- [ ] I've communicated my changes to consumers by [updating changelogs
for packages I've
changed](https://github.com/MetaMask/core/tree/main/docs/processes/updating-changelogs.md)
- [ ] I've introduced [breaking
changes](https://github.com/MetaMask/core/tree/main/docs/processes/breaking-changes.md)
in this PR and have prepared draft pull requests for clients and
consumer packages to resolve them

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds provider fetching and state management across the ramps stack.
> 
> - **RampsService**: New `getProviders(regionCode, { provider, crypto,
fiat, payments })` calling `v2/regions/:regionCode/providers`; validates
responses; exposed via messenger (`RampsService:getProviders`).
> - **RampsController**: New `providers` in state and `getProviders()`
with request caching and region normalization; integrates into `init()`,
`updateUserRegion()`, and `setUserRegion()`; clears `providers` (and
`tokens`) when region changes or fetch fails; persisted and included in
metadata.
> - **Tests**: Comprehensive tests for provider API, controller
integration, caching, filters, and state snapshots; selector tests
updated; changelog updated.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
cca85b7. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: georgeweiler

packages/ramps-controller/CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add `defaultAmount` and `quickAmounts` fields to the `Country` type ([#7645](https://github.com/MetaMask/core/pull/7645))
 
+- Add `providers` state and `getProviders()` method to RampsController. Providers are automatically fetched on init and when the region changes ([#7652](https://github.com/MetaMask/core/pull/7652))
+
 ### Changed
 
 - **BREAKING:** Change `userRegion` from `string | null` to `UserRegion | null`. Access region code via `userRegion.regionCode`. ([#7646](https://github.com/MetaMask/core/pull/7646))