test: add comprehensive config parsing tests and graceful degradation coverage

[clawdeeo]

Adds comprehensive test coverage for config extraction using real TOML and YAML parsers. Includes real-world config fixtures for Python, JavaScript, Go, and Rust ecosystems. Validates graceful degradation when parser dependencies are unavailable and tests invalid config handling.

Changes

• Add real-world config fixtures (pyproject.toml, prettier.yaml, eslint.yaml, golangci.yml, rustfmt.toml, clippy.toml)
• Add tests for TOML/YAML parsing with real fixtures
• Add tests for Rust TOML configs
• Add tests for full detection from fixtures
• Add tests for invalid config handling
• Add tests for mixed-format repos

Test coverage

• Valid TOML/YAML parsing
• Invalid config handling (empty dict returned)
• Parser unavailability (empty dict returned)
• Cross-ecosystem detection
• Real-world config fixtures

86 tests pass.