Improve test suite quality and coverage

[kainlite]

This commit enhances the test suite with multiple improvements:

High Priority Fixes:

• Fixed weak assertions in configuration_tests.rs (lines 305-311, 416-434)
  • Added proper state verification with detailed error messages
  • Improved test assertions to check for specific success/failure conditions
  • Added validation that disabled and invalid deployments are properly filtered

New Test Coverage:

• Added tests for custom branch configuration (observe_branch annotation)
• Added tests for short/long tag type handling and defaults
• Added tests for registry configuration (URL, secret name, namespace)
• Added tests to verify default values (master branch, long tag type)
• Added test for invalid tag type defaulting to long

Code Quality Improvements:

• Created shared test fixtures module (tests/common/mod.rs)
  • minimal_valid_annotations() helper
  • complete_valid_annotations() helper
  • create_test_deployment() helper
  • dummy_ssh_key() constant
• Added comprehensive documentation to all test functions
  • Documented git_tests.rs test cases
  • Documented files_tests.rs test cases
  • Documented configuration_tests.rs test cases

Benefits:

• Reduces code duplication across test files
• Improves test reliability with stronger assertions
• Better test documentation for maintainability
• Increased test coverage for edge cases and default values
• More descriptive failure messages for easier debugging

Related to test analysis and improvement recommendations.