vtgate: Use the time zone setting correctly

[systay]

Description

This pull request addresses a critical issue in the handling of time zone settings within our system. The root cause of the problem was that the time zone system setting was not being correctly fetched and interpreted, which led to it being ignored when evaluating the now() function.

Specifically, the issue arose because we were storing the time zone variable value using a literal SQL expression. This meant that when we tried to use this value, it was still in its raw, encoded form, rather than being properly decoded into a usable time zone string.

Related Issue(s)

Fixes #16820

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.51%. Comparing base (95f2e3e) to head (c4d53fd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16824      +/-   ##
==========================================
- Coverage   69.51%   69.51%   -0.01%     
==========================================
  Files        1569     1569              
  Lines      202517   202520       +3     
==========================================
+ Hits       140780   140782       +2     
- Misses      61737    61738       +1     

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[deepthi]

Can you explain why the fix works? It seems to be moving some code from SafeSession to VCursorImpl but it's unclear why that is correct.
Also, I'm surprised that unit tests have been deleted. It seems to me that they should move into vcursor_impl_test instead.

[systay]

Updated the description 👍