[Query Throttler] Add dry-run mode

[siddharth16396]

Description

This PR introduces option for having a "Dry Run Mode" in the Query Throttler system for Vitess tablets outlined in RFC #18412 , First PR :#18449
In this PR we are Implements a dry-run mode for the Query Throttler that allows observing throttling decisions without actually blocking queries.

Background:

• Query throttling is critical for database performance but can be risky when first deployed
• Operators need visibility into throttling behavior before enabling actual query blocking
• Testing throttling logic in production environments requires non-disruptive monitoring

Solution:

• Add DryRun boolean field to Config struct for controlling dry-runbehavior
• When dry-run is enabled, throttling decisions are logged as warnings but queries proceed normally
• Maintains full compatibility with existing throttling logic and strategies
• Added comprehensive test coverage for all dry-run scenarios including disabled throttler, normal mode, and dry-run mode combinations.

Test Plan:

• Added unit tests covering all dry-run mode scenarios in TestIncomingQueryThrottler_DryRunMode
• Tests verify proper logging behavior and query execution flow
• Existing query throttler tests continue to pass ensuring no regression

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 67.53%. Comparing base (c024fff) to head (c58db58).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18657      +/-   ##
==========================================
- Coverage   67.53%   67.53%   -0.01%     
==========================================
  Files        1613     1613              
  Lines      263780   263784       +4     
==========================================
- Hits       178144   178141       -3     
- Misses      85636    85643       +7     

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[siddharth16396]

We will soon be adding support for "metrics" in throttler, where we will be emitting a metric for this as well.
But that is expected later on.