Fix JSON function tests: resolve UDF schema qualification and JDK 8 compatibility issues

[Ananya2]

Overview
This PR fixes critical issues in JSONFunctionTest.java that were causing test failures due to SQL syntax errors and Java version compatibility problems.

Issues Fixed

1. SQL Syntax Errors in UDF Tests
   Problem: Tests testJsonInputOutputWithUdf and testUdfReturningJson were failing with "Incorrect syntax near 'id'" and "Incorrect syntax near 'data'" errors
   Root Cause: RandomUtil.getIdentifier() intentionally adds single quote characters (') to test identifiers for edge case testing, but SQL Server function calls cannot handle escaped identifiers with special characters
   Solution:
   Replaced generated identifiers with hardcoded schema-qualified function names: "dbo.GetAgeFromJson" and "dbo.GetPersonJson"

2. JDK 8 Compatibility Issue
   Problem: generateHugeJsonFile() method used String.repeat() which was introduced in Java 11, causing compilation failures on JDK 8
   Solution: Replaced String.repeat(1000) with a StringBuilder loop to maintain Java 8 compatibility

3. Test Suite Performance Optimization
   Problem: Large JSON payload tests testInsert1GBJson, testInsertHugeJsonData, testInsert2GBData create multi-GB files causing excessive test execution time
   Solution: Added @disabled annotations to these tests while preserving them for manual execution when needed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.38%. Comparing base (9c41975) to head (28e3a2e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2770      +/-   ##
============================================
+ Coverage     52.30%   52.38%   +0.07%     
+ Complexity     4171     4161      -10     
============================================
  Files           149      149              
  Lines         34241    34241              
  Branches       5718     5718              
============================================
+ Hits          17910    17936      +26     
+ Misses        13874    13838      -36     
- Partials       2457     2467      +10     

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