feat(multichain-account-service): populate maxConcurrency for BTC and TRX providers (#7052)

## Explanation

<!--
Thanks for your contribution! Take a moment to answer these questions so
that reviewers have the information they need to properly understand
your changes:

* What is the current state of things and why does it need to change?
* What is the solution your changes offer and how does it work?
* Are there any changes whose purpose might not obvious to those
unfamiliar with the domain?
* If your primary goal was to update one package but you found you had
to update another one along the way, why did you do so?
* If you had to upgrade a dependency, why did you do so?
-->

## References

<!--
Are there any issues that this pull request is tied to?
Are there other links that reviewers should consult to understand these
changes better?
Are there client or consumer pull requests to adopt any breaking
changes?

For example:

* Fixes #12345
* Related to #67890
-->

## Checklist

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

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Sets a default maxConcurrency of 3 for Bitcoin and Tron account
providers, updates tests and changelogs, and reverts
seedless-onboarding-controller to 6.0.0.
> 
> - **Multichain Account Service**:
> - **Providers**: Set default `maxConcurrency: 3` in
`BtcAccountProvider` and `TrxAccountProvider` constructors
(`src/providers/{Btc,Trx}AccountProvider.ts`).
> - **Tests**: Make `maxConcurrency` optional and add test asserting
default `Infinity` when unspecified
(`src/providers/SnapAccountProvider.test.ts`).
> - **Changelog**: Document BTC/TRX concurrency limit under `Unreleased`
(`packages/multichain-account-service/CHANGELOG.md`).
> - **Seedless Onboarding Controller**:
> - **Version/Changelog**: Revert to `6.0.0`, remove `6.1.0` section,
and update comparison links (`package.json`, `CHANGELOG.md`).
> - **Repo**:
>   - Bump root `package.json` version to `659.0.0`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
ece5cd3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Charly Chevalier <charly.chevalier@consensys.net>

Author: mathieuartu

packages/multichain-account-service/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Limit Bitcoin and Tron providers to 3 concurrent account creations by default when creating multichain account groups ([#7052](https://github.com/MetaMask/core/pull/7052))
+
 ## [2.1.0]
 
 ### Added

packages/multichain-account-service/src/providers/BtcAccountProvider.ts

@@ -28,6 +28,7 @@ export class BtcAccountProvider extends SnapAccountProvider {
   constructor(
     messenger: MultichainAccountServiceMessenger,
     config: BtcAccountProviderConfig = {
+      maxConcurrency: 3,
       createAccounts: {
         timeoutMs: 3000,
       },

packages/multichain-account-service/src/providers/SnapAccountProvider.test.ts

@@ -18,7 +18,7 @@ const TEST_SNAP_ID = 'npm:@metamask/test-snap' as SnapId;
 const TEST_ENTROPY_SOURCE = 'test-entropy-source' as EntropySourceId;
 
 // Helper to create a tracked provider that monitors concurrent execution
-const createTrackedProvider = (maxConcurrency: number) => {
+const createTrackedProvider = (maxConcurrency?: number) => {
   const tracker: {
     startLog: number[];
     endLog: number[];
@@ -67,7 +67,7 @@ const createTrackedProvider = (maxConcurrency: number) => {
 
   const messenger = getMultichainAccountServiceMessenger(getRootMessenger());
   const config = {
-    maxConcurrency,
+    ...(maxConcurrency !== undefined && { maxConcurrency }),
     createAccounts: {
       timeoutMs: 5000,
     },
@@ -197,5 +197,26 @@ describe('SnapAccountProvider', () => {
       // With maxConcurrency=1, never more than 1 should run at a time
       expect(tracker.maxActiveCount).toBe(1);
     });
+
+    it('defaults to Infinity when maxConcurrency is not provided', async () => {
+      const { provider, tracker } = createTrackedProvider();
+
+      // Start 4 concurrent calls
+      const promises = [0, 1, 2, 3].map((index) =>
+        provider.createAccounts({
+          entropySource: TEST_ENTROPY_SOURCE,
+          groupIndex: index,
+        }),
+      );
+
+      await Promise.all(promises);
+
+      // All 4 operations should complete
+      expect(tracker.startLog).toHaveLength(4);
+
+      // Without maxConcurrency specified, should default to Infinity (no throttling)
+      // So all 4 should have been able to run concurrently
+      expect(tracker.maxActiveCount).toBe(4);
+    });
   });
 });

packages/multichain-account-service/src/providers/TrxAccountProvider.ts

@@ -29,6 +29,7 @@ export class TrxAccountProvider extends SnapAccountProvider {
   constructor(
     messenger: MultichainAccountServiceMessenger,
     config: TrxAccountProviderConfig = {
+      maxConcurrency: 3,
       discovery: {
         timeoutMs: 2000,
         maxAttempts: 3,