Explicit ${env:VAR} interpolation support in ConfigLoader

[aidankhogg]

Motivation

• Make environment-variable interpolation behavior explicit and user-friendly by supporting a documented ${env:VAR} alias while retaining OmegaConf's built-in ${oc.env:VAR} syntax.
• Ensure interpolation is reliably available when loading/merging configs and when resolving variables programmatically.
• Provide tests and documentation so callers know how missing variables are handled.

Description

• Add a _get_required_env_var helper and register_env_resolver() that registers a custom OmegaConf resolver for the ${env:VAR} alias and call it at package init and before resolving configs.
• Invoke register_env_resolver() from ConfigLoader.load_config and ConfigLoader.resolve_env_vars, and update their docstrings to document supported syntaxes (${env:VAR} and ${oc.env:VAR}) and missing-variable behavior.
• Add tests in tests/test_config.py that verify ${env:VAR} resolution, ${oc.env:VAR} compatibility, failure on missing variables, and resolve_env_vars behavior using temporary environment variables.
• Run automatic formatting with black on the changed files.

Testing

• Ran pytest tests/test_config.py -q and all tests in that file passed (51 passed).
• Ran python -m black --check netengine/config/loader.py tests/test_config.py which succeeded (files formatted where necessary).
• Ran python -m mypy netengine --strict which reported pre-existing typing issues in unrelated files (these failures are not introduced by this change).

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Codex Task