test: improve code coverage from 94.3% to 98.8%

[miclle]

Summary

This PR significantly improves the test coverage of the fox framework from 94.3% to 98.8%.

Changes

Coverage Improvements by Package

• github.com/fox-gonic/fox: 94.3% → 98.8% (+4.5%)
• github.com/fox-gonic/fox/logger: 90.8% → 91.8% (+1.0%)
• github.com/fox-gonic/fox/httperrors: 96.8% (maintained)
• github.com/fox-gonic/fox/utils: 100.0% (maintained)

Key Improvements

1. binding.go - bind() function (93.2% → 94.9%)

• Add TestBind_CustomBinder for custom binder error path
• Add TestBind_ContextFieldNotConvertible for type conversion edge cases
• Add TestBind_WithBindingError for JSON parsing errors
• Add TestBind_DefaultBinder tests for fallback binding scenarios
• Add TestBind_BodyBinder tests for empty body handling
• Add TestBind_PointerToPointer for nested pointer types
• Add TestBind_NonStructTarget for non-struct binding
• Add TestBind_RequestBodyError for body read errors

2. httperrors/errors.go - MarshalJSON() (92.3% → 96.8%)

• Add TestMarshalJSON_MetaNil for nil Meta field
• Add TestMarshalJSON_CodeEmpty for empty Code field
• Add TestMarshalJSON_ErrorEmpty for empty error message
• Add TestMarshalJSON_MetaCodeInStruct for code in Meta struct
• Add TestMarshalJSON_MetaErrorInStruct for error in Meta struct
• Add TestMarshalJSON_MetaWithMap for Meta with map values
• Add TestMarshalJSON_MetaWithPrimitiveType for Meta with primitive types

3. logger/logger.go - newLogger() (94.7% → 100%)

• Add TestNewLogger_FileLoggingHumanReadable for file logging with human-readable format

4. Other improvements

• Add debug_test.go with comprehensive debug mode tests
• Add context_test.go tests for RequestBody edge cases
• Add render_test.go tests for renderError with render.Render interface
• Add engine_test.go tests for SetMode/Mode and CORS configuration
• Add routergroup_test.go tests for preset logger and trace ID scenarios

Test Results

All tests pass successfully:

make test

Remaining Uncovered Code

The remaining uncovered code consists of:

• Fatal/Fatalf functions in logger package (cannot be safely tested as they exit the program)
• Panic/Panicf functions in logger package (not recommended to test)
• Error handling paths in bind() that are extremely difficult to trigger (Query/URI/Header binding errors)

These represent edge cases that are either unsafe to test or nearly impossible to trigger in practice.

Checklist

• All tests pass
• Coverage improved significantly
• No breaking changes
• Focused on testing real-world scenarios and edge cases

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.00%. Comparing base (56ab157) to head (1d95a29).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   91.87%   96.00%   +4.13%     
==========================================
  Files          16       16              
  Lines         726      726              
==========================================
+ Hits          667      697      +30     
+ Misses         42       22      -20     
+ Partials       17        7      -10     

Flag	Coverage Δ
unittests	96.00% <ø> (+4.13%)	⬆️

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see 6 files with indirect coverage changes

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