[CICD] flagos user tests management

[Darryl233]

Summary

Add flagos-user-tests subdirectory with a standardized user test framework supporting multi-repo test case management (flagscale, flagcx, flaggems, etc.)

• Provide a complete test toolchain: test case collection (collect_test_cases.py), matrix resolution (resolve_matrix.py), test execution (run_user_tests.py), and Conda environment initialization (activate_conda.sh)
• Add validator tools (validators/): config validation, gold values validation, and lint checks to enforce test case format conventions
• Provide a test case template generator (generators/create_test_template.py) to simplify authoring new test cases
• Add CI workflow post_test_cases.yml that automatically collects test case metadata and uploads it to the backend via the post-benchmark-report action after PRs are merged to main, enabling continuous tracking of the test case inventory
• Include flagscale/inference/qwen3/demo_0_6b as a reference test case implementation
• Add comprehensive documentation: README, CONTRIBUTING guide, test format spec (test_format_spec.md), and getting started guide (getting_started.md)
• Fix report JSON output format in collect_test_cases.py to use object-of-objects structure required by post-benchmark-report action (was incorrectly outputting a JSON array)