fix: json encoding on bind variables

[harshit-gangal]

Description

This PR fixes the issue with sending json type as bind variable to vttablet.

Related Issue(s)

• Fixes Bug Report: syntax error while trying to use INSERT INTO ... SELECT * FROM ... to copy data between keyspaces #16614

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

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 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

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

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]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.87%. Comparing base (3c2e8f9) to head (068a651).
Report is 23 commits behind head on main.

Files	Patch %	Lines
go/vt/vtgate/engine/insert_select.go	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16615      +/-   ##
==========================================
+ Coverage   68.77%   68.87%   +0.10%     
==========================================
  Files        1557     1562       +5     
  Lines      199826   200627     +801     
==========================================
+ Hits       137422   138178     +756     
- Misses      62404    62449      +45     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[harshit-gangal]

Closing in favor of #16637