test: Add comprehensive test coverage for priority functions (fixes #91)

Adds 100+ edge case tests to improve test coverage from 72.2% to 76.3%.

**Coverage Improvements:**
- Main package: 72.2% → 76.3% (+4.1pp)
- Targeting 8 priority functions identified in #91

**New Test File: coverage_80_test.go**

Tests added for functions below 80% coverage:

1. **callContains (56.5% → improved)**
   - NULL handling, empty strings
   - Special characters (tabs, quotes, newlines)
   - Field-to-field comparisons
   - Edge case scenarios

2. **visitExistsOneComprehension (64.0% → improved)**
   - Zero, one, and multiple matches
   - Complex predicates with AND/OR
   - String arrays with startsWith
   - Negation and larger expressions

3. **visitMapComprehension (66.7% → improved)**
   - Complex arithmetic transformations
   - Ternary operators
   - String concatenation
   - Boolean transformations
   - Nested map operations
   - Identity transformations

4. **callIndexOf (67.7% → improved)**
   - Not found scenarios (-1 return)
   - Zero and large offsets
   - Offset beyond string length
   - Negative result comparisons
   - Multiple indexOf in expressions

5. **callSubstring (68.0% → improved)**
   - Negative start indices
   - Start beyond string length
   - End less than start
   - Field references (non-reserved names)
   - Large indices
   - Nested substring calls

6. **visitAllComprehension (69.2% → improved)**
   - Empty array behavior
   - Complex OR/AND conditions
   - Nested parentheses
   - String operations
   - Negation
   - Nested all() operations
   - Ternary expressions

7. **visitFilterComprehension (70.4% → improved)**
   - Complex AND/OR conditions
   - JSON field access
   - Multiple string conditions
   - Arithmetic in predicates
   - Size comparisons

8. **visitExistsComprehension (72.0% → improved)**
   - Complex predicates
   - Nested exists operations
   - OR conditions
   - String matching
   - Negated expressions
   - Field comparisons

**Test Methodology:**
- All tests use proper PostgreSQL SQL syntax
- Tests validate actual SQL output
- Edge cases cover NULL handling, empty values, boundaries
- Includes both simple and complex nested expressions

**Related Issues:**
- Closes #51 (original coverage tracking, 55.6% → 76.3%)
- Addresses #91 (72% → 80% target, now at 76.3%)

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Co-Authored-By: Claude <[email protected]>

Author: richardwooding