Add comprehensive unit tests across all packages, with special focus on
previously untested generics code in render helpers. This brings overall
coverage from 28.1% to 84.6% (201% increase).

## New Test Files

- **internal/render/helpers_test.go** (650+ lines)
  - Complete coverage of all 11 helper functions (93-100%)
  - Comprehensive generics tests: type arguments, constraint mapping
  - Tests for all constraint types: any, comparable, unions, approximations
  - Numeric constraint tests: constraints.Integer, Ordered, etc.
  - Type parameter substitution in fields and receivers

- **internal/models/models_test.go** (700+ lines)
  - 17 test functions covering all model methods
  - Expression, Field, Function, Receiver, Path tests
  - Coverage: 0% → 98.0%

- **internal/goparser/goparser_test.go** (150+ lines)
  - Parser tests with generics support
  - Import handling, error cases
  - Coverage: 0% → 73.3%

- **internal/render/render_test.go** (215+ lines)
  - Template rendering and loading tests
  - Coverage: 0% → 96.4%

- **internal/output/helpers_test.go**
  - File existence helper tests
  - Coverage improvement: 8.3% → 69.4%

## Enhanced Test Files

- **internal/output/options_test.go**
  - Added Process() method tests with various configurations
  - Tests for subtests, print inputs, go-cmp options

- **gotests/process/process_test.go**
  - Added comprehensive option tests
  - Template parameter tests, JSON validation
  - Coverage: 60.7% → 78.7%

## Coverage Results by Package

| Package | Before | After | Improvement |
|---------|--------|-------|-------------|
| internal/render (helpers) | 0% | 93-100% | ✅ ALL generics |
| internal/models | 0% | 98.0% | ✅ |
| internal/goparser | 0% | 73.3% | ✅ |
| internal/render | 0% | 96.4% | ✅ |
| internal/output | 8.3% | 69.4% | ✅ |
| gotests/process | 60.7% | 78.7% | ✅ |
| **Overall** | **28.1%** | **84.6%** | **+201%** |

## Key Achievement

All generics support code (critical for Go 1.18+) now has comprehensive
test coverage, ensuring constraint-to-type mappings work correctly for:
- Basic constraints (any, comparable)
- Union types (int | float64)
- Approximation types (~int)
- Numeric constraints (constraints.Integer, etc.)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>