[14.0][IMP] connector_extension: add server_tz field to backend

[eantones]

Summary

• Add server_tz field to connector.extension.backend for backends where the server timezone differs from the data timezone
• Add server_tz_to_utc and server_tz_to_local conversion methods with fallback to tz when not set
• Enables correct handling of technical timestamps (e.g., modification dates) that follow the server TZ while business data follows the data TZ

Test plan

• Verify existing connectors still work without server_tz set (fallback to tz)
• Set server_tz on a backend and verify server_tz_to_local/server_tz_to_utc use the correct timezone

[codecov]

Codecov Report

❌ Patch coverage is 28.57143% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.75%. Comparing base (7c24d82) to head (87efe13).
⚠️ Report is 3 commits behind head on 14.0.

Files with missing lines	Patch %	Lines
connector_extension/models/backend/backend.py	30.76%	8 Missing and 1 partial ⚠️
connector_extension_sql/components/adapter.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             14.0     #884      +/-   ##
==========================================
+ Coverage   50.71%   50.75%   +0.04%     
==========================================
  Files         979      979              
  Lines       16752    16756       +4     
  Branches     3566     3567       +1     
==========================================
+ Hits         8495     8505      +10     
+ Misses       8039     8034       -5     
+ Partials      218      217       -1     

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