Fix XPath boolean result output

[sjh9714]

Summary

Fixes XPath boolean query results.

What changed

• Added regression coverage for boolean(...) XPath results.
• Updated XPathQuery to print boolean values instead of treating them as unknown result types.
• Preserved existing number, string, and node iterator output handling.

Why

Issue #188 reports that xq -x 'boolean(//root)' fails with unknown type error: true instead of printing true.\n\n## Testing\n\n- go test ./internal/utils -run TestXPathQueryBoolean\n- go test ./internal/utils\n- go test ./...\n\nFixes #188

[sibprogrammer]

@sjh9714 Thank you!

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.78%. Comparing base (a4e001c) to head (a0ce12f).
⚠️ Report is 2 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   80.73%   80.78%   +0.05%     
==========================================
  Files           5        5              
  Lines         711      713       +2     
==========================================
+ Hits          574      576       +2     
  Misses         94       94              
  Partials       43       43              

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