Fix: a0deploy keyword replacement mappings not applied to theme.json

[ankita10119]

Prior to this fix, keyword placeholder substitution (e.g. ##CDN_URL##) was silently skipped for theme assets during export. The root cause was that ThemesHandler did not declare an identifiers array, so the keyword-preservation logic - which uses identifiers to build dot-notation lookup paths for each array item - found no matching fields and returned the raw resolved remote URLs instead of preserving the original placeholders.

🔧 Changes

Added identifiers: ['themeId'] to the DefaultHandler options in ThemesHandler, aligning it with how other handlers expose their identifier field so that getPreservableFieldsFromAssets can correctly locate each theme record.

🔬 Testing

New unit tests added under describe('#themes keyword preservation') in test/tools/auth0/handlers/themes.tests.js:

1. Positive case – verifies that when themeId is included in identifiers, keyword placeholders (e.g. ##CDN_URL##) in nested theme fields (fonts.font_url, widget.logo_url) are preserved after export.
2. Regression case – verifies that when identifiers does not include themeId (the pre-fix state), placeholders are not preserved and raw URLs are returned, documenting exactly why the bug occurred.

📝 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 79.85%. Comparing base (406a6fc) to head (f126ae2).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1379   +/-   ##
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  Coverage   79.85%   79.85%           
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  Files         153      153           
  Lines        7023     7023           
  Branches     1537     1537           
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  Hits         5608     5608           
  Misses        780      780           
  Partials      635      635           

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