[US-054-T3] Add edge case tests and finalize validation

[KochTobi-Agent]

Task Description

Add edge case tests and finalize the bundle validation feature.

TDD Cycle

RED Phase - Write Failing Tests

Create edge case tests:

• TestValidateBundle_EmptyPresetsArray - empty presets should fail (minItems: 1 in schema)
• TestValidateBundle_DeeplyNestedBundle - validation works with nested directory structures
• TestValidateBundle_MalformedArchive - graceful handling of corrupted tar.gz
• TestValidateBundle_PermissionDenied - handles permission errors gracefully
• TestBundleValidateCommand_VerboseOutput - verbose flag shows detailed validation info
• TestValidateBundle_MultipleErrors - reports all validation errors, not just first

GREEN Phase - Implement to Pass

• Enhance ValidateBundle() to handle edge cases
• Add --verbose flag to CLI command (optional)
• Ensure all errors are collected and reported (not fail-fast)
• Add robust error messages for edge cases
• Integration testing with real bundle fixtures

Finalization Tasks

• Run full test suite: go test ./...
• Manual testing with real bundles
• Documentation update (if needed)
• Code review preparation

Acceptance Criteria

• All edge case tests pass
• Integration tests pass
• Feature works end-to-end:
  • go run . bundle validate ./e2e/testdata/fixture-bundle exits 0
  • Invalid bundles report all errors with descriptive messages
• Code follows project conventions
• Ready for PR