fix(ci): replace Poetry runtime setup with uv

[aviralgarg05]

Summary

This PR removes the runtime dependency on Poetry from the Python build path and switches the backend, test, and container workflows to uv.

The change is intentionally narrow:

• added a small shared helper to bootstrap uv, create or reuse project-local virtual environments, and run Python or pytest commands consistently
• updated the Azure DevOps and fake plugin build and run scripts to use that helper instead of Poetry
• updated the backend Python test runner to use the same flow and to avoid collecting tests from .venv directories
• updated the backend image and the lake-builder image to install uv instead of Poetry
• fixed the backend image copy step so the non-root devlake user can create project-local virtual environments during image builds

This addresses the CI and Docker breakage caused by the Poetry-based Python 3.9 setup, especially in cross-platform and emulated build paths.

Does this close any open issues?

Closes #8776

Screenshots

No UI changes.

Other Information

Validation completed against the updated flow:

• make build-python passed in the CI-style mericodev/lake-builder:latest container
• make unit-test-python passed in the same container
• docker buildx build --platform linux/amd64 --target base ... backend passed
• docker build --target base ... backend passed on the native platform

Relevant test results:

• Azure DevOps Python tests: 8 passed, 1 skipped
• PyDevLake Python tests: 10 passed

Suggested labels:

• pr-type/bug-fix

[narrowizard]

Sorry, I'm not familiar with Python package management or DevLake's Python plugins. I only opened the issue because I encountered a timeout problem at that time, and I don't have the capability to review this modification. @klesh @aviralgarg05

[klesh]

@keon94 Would you like to take a look?