Delegate Column Availability Checks to MySQL for Single-Route Queries

[harshit-gangal]

Description

This PR modifies vtgate to bypass column availability checks when a query can be routed to a single shard, allowing MySQL to handle column validation directly. This prevents unnecessary ambiguous column errors at vtgate when schema tracking is disabled.

Related Issue(s)

Fixes #17083

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]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.17%. Comparing base (17607fa) to head (f82991b).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17077      +/-   ##
==========================================
+ Coverage   67.14%   67.17%   +0.02%     
==========================================
  Files        1571     1571              
  Lines      252060   252246     +186     
==========================================
+ Hits       169249   169440     +191     
+ Misses      82811    82806       -5     

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[harshit-gangal]

new changes looks good.