fix: remove enable_custom_domain_in_emails from managed tenant flags

[ankita10119]

🔧 Changes

Removes enable_custom_domain_in_emails from the managed tenant flags allowlist (allowedTenantFlags) to prevent import failures in environments without a ready custom domain.

Problem: The flag was included in allowedTenantFlags, causing it to be:

1. Included in export output (tenant.yaml) even though it is deprecated
2. Sent to the Management API during import, resulting in a 400 Bad Request error on tenants that do not have a custom domain with status: "ready" - aborting the entire tenant processChanges stage and leaving no tenant settings applied

Changes:

• Removed enable_custom_domain_in_emails from allowedTenantFlags in src/tools/auth0/handlers/tenant.ts - the existing removeUnallowedTenantFlags() function now strips it on both the export and import paths automatically
• Updated the deprecation warning message to accurately state the flag is no longer applied, and directs users to migrate to customDomains[].is_default
• Updated unit test to assert the flag is absent from the Management API payload, not just that the warning is logged
• Removed the flag from three e2e testdata fixtures that used it as import input

Migration: Users should remove enable_custom_domain_in_emails from their tenant configuration files and use the is_default field on customDomains instead:

customDomains:
  - domain: mail.example.com
    type: auth0_managed_certs
    tls_policy: recommended
    is_default: true

📚 References

• Supersedes the deprecation warning introduced in feat: Add support for default custom domain and deprecation warning for enable_custom_domain_in_emails  #1330 -the flag is now fully removed from management rather than just warned about

🔬 Testing

Unit tests: Updated test in test/tools/auth0/handlers/tenant.tests.ts verifies:

• Deprecation warning is still logged when the flag is present in input
• The flag is not present in the payload sent to client.tenants.settings.update()

Manual export test: Running node lib/index.js export against a tenant with enable_custom_domain_in_emails: true confirms:

• The flag no longer appears in the exported tenant.yaml
• The export log lists the flag under the existing "incompatible flags" warning

Not tested end-to-end for import (requires a tenant without a custom domain): the import path is covered by the unit test and by code analysis - removeUnallowedTenantFlags() strips the flag before the API call in processChanges().

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.10%. Comparing base (f7873f7) to head (901507a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1401   +/-   ##
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  Coverage   80.10%   80.10%           
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  Files         153      153           
  Lines        7099     7099           
  Branches     1566     1566           
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  Hits         5687     5687           
  Misses        760      760           
  Partials      652      652           

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