Adds support to GET/PATCH default Custom domain

[kailash-b]

Changes

Adds support for the GET and PATCH /custom-domains/default Management API endpoints, allowing callers to retrieve and set the default custom domain for a tenant.

Endpoints added:

• GET /api/v2/custom-domains/default — retrieves the default custom domain
• PATCH /api/v2/custom-domains/default — sets the default custom domain

Classes and methods added:

• ICustomDomainsClient.GetDefaultAsync() — retrieves the default CustomDomain for the tenant
• ICustomDomainsClient.SetDefaultAsync(CustomDomainSetDefaultRequest) — sets the default custom domain by domain name
• CustomDomainSetDefaultRequest — new request model with a Domain property

Usage example:

// Get the default custom domain
var defaultDomain = await managementClient.CustomDomains.GetDefaultAsync();

// Set a custom domain as the default
var updated = await managementClient.CustomDomains.SetDefaultAsync(
    new CustomDomainSetDefaultRequest { Domain = "login.example.com" });

Testing

• This change adds unit test coverage

• This change adds integration test coverage

• This change has been tested on the latest version of the platform/language or why not

Checklist

• I have read the Auth0 general contribution guidelines

• I have read the Auth0 Code of Conduct

• All existing and new tests complete without errors

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.17%. Comparing base (f979eff) to head (2d991d8).

Files with missing lines	Patch %	Lines
...agementApi/Models/CustomDomain/CustomDomainBase.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #958      +/-   ##
==========================================
- Coverage   80.21%   80.17%   -0.04%     
==========================================
  Files         464      465       +1     
  Lines        6045     6053       +8     
  Branches      293      293              
==========================================
+ Hits         4849     4853       +4     
- Misses       1104     1108       +4     
  Partials       92       92              

Flag	Coverage Δ
authIntTests	30.64% <0.00%> (-0.05%)	⬇️
mgmtIntTests	61.27% <87.50%> (+1.06%)	⬆️
unittests	18.23% <0.00%> (-0.04%)	⬇️

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[tanya732]

LGTM