Fix quickstart download failing with "zip: not a valid zip file" error

[ramya18101]

🔧 Changes

• Updated to use standard CLI User-Agent format for request of quickstarts DownloadLink
• Enhanced content-type validation with clearer error messages when receiving unexpected content types

📚 References

• Integration tests failed revealed the auth0 docs server behaviour varies based on User-Agent header

🔬 Testing

Verify consistent successful downloads without "zip: not a valid zip file" errors

📝 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

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.53%. Comparing base (72153d5) to head (9fb3fd4).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/auth0/quickstart.go	57.14%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1372      +/-   ##
==========================================
+ Coverage   64.51%   64.53%   +0.02%     
==========================================
  Files         108      108              
  Lines       17959    18048      +89     
==========================================
+ Hits        11586    11648      +62     
- Misses       5615     5634      +19     
- Partials      758      766       +8     

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