Add comprehensive unit tests with 63.9% coverage

Implemented unit tests across all internal packages to achieve
63.9% test coverage, exceeding the 60% target.

Test coverage by package:
- internal/config: 100.0% (config validation, constants)
- internal/metrics: 100.0% (Sample methods, Collectors, Simulate)
- internal/parser: 92.3% (CSV/JSON parsing, format detection)
- internal/ingester: 44.9% (auto routing, time series conversion)

New test files:
- internal/config/config_test.go: Config creation and constants
- internal/metrics/sample_test.go: Sample type methods (Age, IsRecent)
- internal/metrics/generator_test.go: Collectors and simulation
- internal/parser/csv_test.go: CSV parsing with various inputs
- internal/parser/json_test.go: JSON parsing and validation
- internal/parser/parser_test.go: Parser factory and format handling
- internal/ingester/auto_test.go: Auto mode routing logic
- internal/ingester/remotewrite_test.go: Time series conversion
- internal/ingester/pushgateway_test.go: Pushgateway ingester

Tests cover:
- Happy path and error cases
- Context cancellation support
- Edge cases (empty input, invalid formats)
- Label parsing and timestamp handling
- Metric type generation (counter, gauge, histogram)
- Table-driven tests for comprehensive coverage

All 50+ tests passing ✅

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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