Merge PackageInstance and PackageTestConfig into one class

[coderabbitai]

Context

During the review of PR #1843, the team discussed whether PackageInstance and PackageTestConfig should remain separate classes.

After discussion, the consensus is to merge them into a single class. The rationale:

• The separation adds an additional layer of abstraction without clear concrete benefits at this stage
• Even with more attributes in a merged class, we can use grouping and docstrings to maintain clarity
• PackageInstance tracks intermediate states/artifacts while PackageTestConfig supplies input configs, but these purposes can coexist in one class

References

• PR: feat(integration-tests): Modularize package tests with per-mode directories and module-scoped fixtures. #1843
• Discussion: feat(integration-tests): Modularize package tests with per-mode directories and module-scoped fixtures. #1843 (comment)
• Requested by: @junhaoliao