test: Add Priority 3 edge case tests for moderate-coverage functions (#51)

Improves coverage from 70.1% to 71.4% (+1.3pp)

Added edgecase_coverage_test.go with comprehensive edge case tests for:

**Major Improvements:**
- visitConst: 72.0% → **96.0%** ✅ (+24pp) - Excellent coverage!
- callEndsWith: 72.2% → **83.3%** ✅ (+11.1pp) - Good improvement!

**Test Coverage Added (78 test cases):**

1. TestConstEdgeCases (17 cases)
   - MAX_INT64, MIN_INT64, zero values
   - Very large/small doubles, scientific notation
   - Empty strings, Unicode, quotes escaping
   - NULL, bool, bytes, uint64 edge values

2. TestStringFunctionEdgeCases (13 cases)
   - contains() with empty strings, special chars
   - startsWith() with %, _, \ (LIKE special chars)
   - endsWith() with LIKE escape sequences
   - Unicode string handling

3. TestBinaryOperatorEdgeCases (15 cases)
   - Bool comparisons (IS/IS NOT)
   - String and list concatenation (||)
   - Modulo operator (MOD function)
   - IN operator with arrays (ANY)
   - Logical operators, nested expressions

4. TestIdentifierEdgeCases (5 cases)
   - Numeric iteration variables
   - Field access patterns

5. TestCallFuncEdgeCases (6 cases)
   - Type conversions (int, double, string, bool)
   - CAST operations

6. TestSpecialDoubleValues (4 cases)
   - Near-infinity values
   - Scientific notation (large and small)

7. TestComplexNestedExpressions (5 cases)
   - Deeply nested arithmetic
   - Complex boolean logic
   - Mixed string and numeric operations
   - Ternary-style expressions

**Analysis:**
- visitConst now has excellent coverage with comprehensive edge cases
- callEndsWith improved significantly with LIKE escape sequence tests
- Some functions (callContains, visitCallBinary, visitCallFunc) need more complex scenarios for further improvement

Total improvement from baseline: 55.6% → 71.4% (+15.8pp)
Progress toward 80% goal: 8.6pp remaining

Author: richardwooding