Fix stack overflow in ToThaiWords(int, bool) extension method

[Copilot]

Issue

The ToThaiWords(this int value, bool useEtWithTensOnly) extension method was causing a stack overflow due to infinite recursion. The method was incorrectly calling itself instead of delegating to the long version.

Root Cause

// Before (line 37 in FormatterExtensions.cs)
public static string ToThaiWords(this int value, bool useEtWithTensOnly) 
    => ToThaiWords(value, useEtWithTensOnly);  // calls itself infinitely!

Solution

// After 
public static string ToThaiWords(this int value, bool useEtWithTensOnly) 
    => ToThaiWords((long)value, useEtWithTensOnly);  // properly delegates to long version

Changes Made

• Fixed infinite recursion by casting int to long and calling the correct overload
• Added regression tests to prevent future occurrences
• Updated test project target framework from net6.0 to net8.0 for compatibility

Verification

• ✅ Stack overflow eliminated - method executes without infinite recursion
• ✅ All existing tests pass (47 tests) + 4 new regression tests = 51 total tests passing
• ✅ Manual testing confirms correct Thai text output:

  42.ToThaiWords(true)   // Returns: "สี่สิบสอง"
  101.ToThaiWords(true)  // Returns: "หนึ่งร้อยหนึ่ง"
  1001.ToThaiWords(true) // Returns: "หนึ่งพันหนึ่ง"

• ✅ Behavior matches the long version exactly
• ✅ No regressions introduced

The fix follows the existing pattern used by other extension methods in the same class and is minimal and surgical.

Fixes #14.

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