feat: Add comprehensive test coverage for C Data Interface

This commit significantly improves test coverage for the C Data Interface
implementation to address the Codecov report showing low patch coverage
(15.65% with 307 missing lines).

## Coverage Improvements Added:

### Export Function Coverage:
- Schema flags testing for all vector types
- Release callback verification and cleanup testing
- Buffer management for primitive and boolean vectors
- Dictionary support validation (returns C_NULL for non-dict vectors)
- Guardian registry lifecycle testing with memory cleanup verification
- Comprehensive primitive type export testing (Int8, Int16, Int32, UInt8, etc.)
- Large array stress testing (1000+ elements)

### Import Function Coverage:
- Method existence verification for all import functions
- Pointer type conversion testing (Ptr{Nothing} compatibility)
- Round-trip testing for all numeric types and boolean vectors
- Nullable type import verification with missing value handling
- Complex type import infrastructure validation

### Format String Utilities Coverage:
- Complete primitive type format string generation testing
- Union/nullable type format string handling
- Complex format string parsing (+l, +s, +w:N patterns)
- Date/Time format string generation (Dates.Date, DateTime)
- Arrow vector-specific format generation testing
- Comprehensive C string utilities testing with edge cases
- Invalid format string error handling
- Null pointer safety verification

### String/Binary Export Coverage:
- ToList string vector export verification
- Binary vector export testing
- Empty array edge case handling
- List buffer structure validation (offsets + data buffers)

### Memory Safety Testing:
- Guardian object registration and cleanup verification
- Release callback execution testing
- Foreign memory management through ImportedArrayHandle

## Test Structure:
- Added 13+ new test sets with 150+ individual test cases
- Covers all major export/import code paths previously untested
- Validates error handling and edge cases
- Ensures memory safety patterns work correctly

This comprehensive test suite should significantly improve the patch coverage
metrics reported by Codecov, addressing the 307 missing lines across:
- src/cdata/export.jl (134 missing lines)
- src/cdata/import.jl (126 missing lines)
- src/cdata/format.jl (33 missing lines)

🤖 Implementation developed with Claude Code assistance
Research and Technical Design: Original contribution

Author: ollemartensson