mgmt: Adds Checkpoint Pagination support for fetching CustomDomains

[kailash-b]

Changes

Adds Checkpoint Pagination support for fetching Custom Domains via the Management API.

Endpoints changed:

• GET /custom-domains — now supports checkpoint-based pagination via an additional overload

Classes and methods added:

• CustomDomainsGetAllRequest — new request model supporting optional Query, Fields, IncludeFields, and Sort parameters
• ICustomDomainsClient.GetAllAsync(CustomDomainsGetAllRequest, CheckpointPaginationInfo?, CancellationToken) — new overload returning ICheckpointPagedList<CustomDomain>
• CustomDomainsClient.GetAllAsync(CustomDomainsGetAllRequest, CheckpointPaginationInfo?, CancellationToken) — implementation of the above

Model changes:

• CustomDomain — added IsDefault (bool?) property mapped to the is_default JSON field

Usage example:

var result = await managementClient.CustomDomains.GetAllAsync(
    new CustomDomainsGetAllRequest { Sort = "domain:1" },
    new CheckpointPaginationInfo(from: null, take: 50)
);


### References

### Testing

- [x] This change adds unit test coverage

- [ ] This change adds integration test coverage

- [x] This change has been tested on the latest version of the platform/language or why not

### Checklist

- [x] I have read the [Auth0 general contribution guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md)

- [x] I have read the [Auth0 Code of Conduct](https://github.com/auth0/open-source-template/blob/master/CODE-OF-CONDUCT.md)

- [x] All existing and new tests complete without errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.21%. Comparing base (d893e05) to head (b6fdbda).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #939      +/-   ##
==========================================
+ Coverage   80.11%   80.21%   +0.09%     
==========================================
  Files         462      464       +2     
  Lines        6025     6045      +20     
  Branches      291      293       +2     
==========================================
+ Hits         4827     4849      +22     
+ Misses       1105     1104       -1     
+ Partials       93       92       -1     

Flag	Coverage Δ
authIntTests	30.68% <5.00%> (-0.09%)	⬇️
mgmtIntTests	60.21% <100.00%> (+0.16%)	⬆️
unittests	18.27% <5.00%> (-0.06%)	⬇️

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[tanya732]

LGTM