MOD-15930 Use devcontainer distro for OS metadata

[ofiryanai]

Summary

Use /etc/devcontainer as the preferred source for the generated operating_systems metadata when a devcontainer image provides a DISTRO value.

Changes

• Add /etc/devcontainer parsing in RAMP/module_metadata.py
• Prefer DISTRO from /etc/devcontainer before falling back to the existing distro package detection
• Add focused unit tests for devcontainer precedence and fallback behavior
• Run the new unit test from tox.ini

Context

Redis module build images can expose the intended package platform in /etc/devcontainer while /etc/os-release may represent a lower-level compatibility base. RAMP should use the devcontainer platform metadata when available so generated module.json matches the platform used by the packaging environment.

Validation:

• python3 -m unittest test_module_metadata.py
• python3 -m py_compile RAMP/module_metadata.py test_module_metadata.py
• git diff --check