fix: expose agent module via cascadeflow.agent attribute

[AasheeshLikePanner]

🎯 Description

This fix exposes the agent module via cascadeflow.agent attribute, which was previously missing from the lazy imports. The test test_top_level_exports_exist was failing because hasattr(cascadeflow.agent, 'PROVIDER_REGISTRY') raised an AttributeError.

🔗 Related Issues

• Related to test_harness_api.py::test_top_level_exports_exist

🔄 Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✅ Test additions/improvements

🧪 Testing

Test cases added

• Unit tests

How to test

import cascadeflow
# Before: AttributeError: module 'cascadeflow' has no attribute 'agent'
# After: Works correctly
assert hasattr(cascadeflow.agent, 'PROVIDER_REGISTRY')

📋 Checklist

Code Quality

• My code follows the project's style guidelines
• I have performed a self-review of my code
• My changes generate no new warnings
• I have run black to format my code
• I have run ruff and fixed all linting issues

Testing

• New and existing unit tests pass locally with my changes
• Tests in test_harness_api.py pass

Breaking Changes

• This PR includes NO breaking changes

🙏 Reviewers

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• I have read and followed the CONTRIBUTING.md guidelines
• My contribution is my own original work or properly attributed
• I agree to license my contribution under the project's MIT license
• I have tested my changes thoroughly