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

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

[vitess-bot[bot]]

Review Checklist

Hello reviewers! :wave: Please follow this checklist when reviewing this Pull Request.

General

• [ ] Ensure that the Pull Request has a descriptive title.
• [ ] Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• [ ] Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• [ ] Apply the release notes (needs details) label if users need to know about this change.
• [ ] New features should be documented.
• [ ] There should be some code comments as to why things are implemented the way they are.
• [ ] There should be a comment at the top of each new or modified test to explain what the test does.

New flags

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If a workflow is added or modified:

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Backward compatibility

• [ ] Protobuf changes should be wire-compatible.
• [ ] Changes to _vt tables and RPCs need to be backward compatible.
• [ ] RPC changes should be compatible with vitess-operator
• [ ] If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• [ ] vtctl command output order should be stable and awk-able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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[systay]

Updated the description 👍