Improve test coverage from 72% to 80% target

[richardwooding]

Current Coverage Status

Latest measurement (Oct 31, 2025):

• Main package: 72.2% statement coverage
• pg package: 91.9% statement coverage
• Gap to 80% target: 7.8 percentage points

This issue tracks the final push to reach our 80% coverage goal. We've already made excellent progress (from 55.6% in issue #51), and now need focused test additions for specific functions.

Priority Functions Needing Coverage

Priority 1: Functions <70% (Biggest Impact)

1. callContains - 56.5% coverage

   • Test JSON array contains with ? operator
   • Test NULL handling in contains operations
   • Test empty string/array edge cases
   • Test contains with special characters needing escaping

2. visitExistsOneComprehension - 64.0% coverage

   • Test zero matches scenario
   • Test exactly one match (happy path)
   • Test multiple matches scenario (should fail/error)
   • Test with empty arrays

3. visitMapComprehension - 66.7% coverage

   • Test complex transformations (arithmetic, string operations)
   • Test nested map() operations
   • Test map with filter conditions
   • Test map with different return types

4. callIndexOf - 67.7% coverage

   • Test offset parameter edge cases
   • Test not-found scenarios (should return -1)
   • Test negative offsets
   • Test offset beyond string length

5. callSubstring - 68.0% coverage

   • Test single-argument version (substring from index to end)
   • Test two-argument version (substring with length)
   • Test negative indices
   • Test out-of-bounds scenarios

6. visitAllComprehension - 69.2% coverage

   • Test with empty arrays
   • Test all true scenario
   • Test all false scenario
   • Test nested all() operations
   • Test all() with complex conditions

Priority 2: Functions 70-79% (Polish)

7. visitFilterComprehension - 70.4% coverage

   • Test complex filter conditions (multiple ANDs/ORs)
   • Test filter with nested field access
   • Test filter on JSON arrays

8. visitExistsComprehension - 72.0% coverage

   • Test exists on empty arrays
   • Test exists with complex predicates
   • Test nested exists() operations

Functions Explicitly Excluded

These functions are intentionally not targeted (0% coverage is acceptable):

• getFieldElementType - Only used in rare ElementType scenarios
• callQuote - Intentionally returns error (not part of CEL standard)
• visitTransformListComprehension - Not reachable by pattern analyzer
• visitTransformMapComprehension - Not implemented
• visitTransformMapEntryComprehension - Not implemented
• visitStructMsg - Not supported in standard CEL

Success Criteria

• Main package coverage reaches 80%+
• All 8 priority functions reach 80%+ individual coverage
• All tests pass with make test
• Code passes linting with make lint
• No regressions in existing functionality

Related Issues

• Closes Test Coverage Gaps - Only 55.6% Coverage #51 (original coverage tracking issue)

[richardwooding]

Progress Update

Significant progress made toward the 80% coverage target!

Coverage Status:

• Starting: 72.2%
• Current: 76.3%
• Improvement: +4.1 percentage points
• Remaining to 80% target: 3.7pp

Work Completed

Created PR #92 with comprehensive test coverage:

• Added coverage_80_test.go with 100+ edge case tests
• All 8 priority functions received targeted test coverage
• All tests pass, code is linted and formatted

Test Coverage Breakdown

New tests added for:

1. ✅ callContains (56.5% → improved) - 7 tests
2. ✅ visitExistsOneComprehension (64.0% → improved) - 8 tests
3. ✅ visitMapComprehension (66.7% → improved) - 10 tests
4. ✅ callIndexOf (67.7% → improved) - 7 tests
5. ✅ callSubstring (68.0% → improved) - 10 tests
6. ✅ visitAllComprehension (69.2% → improved) - 10 tests
7. ✅ visitFilterComprehension (70.4% → improved) - 7 tests
8. ✅ visitExistsComprehension (72.0% → improved) - 6 tests

Next Steps to Reach 80%

To gain the remaining 3.7pp, we could:

1. Add more edge cases for functions currently at 70-79%
2. Test error paths and corner cases more thoroughly
3. Add integration tests for complex multi-function scenarios

The work in PR #92 provides a solid foundation and demonstrates consistent progress toward the goal.