Refactor spec loader validation into shared helper and add schema-version tests

[aidankhogg]

Motivation

• Reduce duplication by centralizing repeated spec validation steps (YAML object checks, metadata/schema_version checks, Pydantic model creation, cross-validation, and feature-state validation) into a single private helper.
• Ensure consistent behavior and error messages across the three public loader entry points (load_spec, load_spec_with_composition, load_spec_with_environment).

Description

• Add _validate_spec_data(data: dict[str, Any], *, validate_feature_states: bool) -> NetEngineSpec which performs type checks on data and metadata, rejects unsupported metadata.schema_version, constructs NetEngineSpec(**data), runs _cross_validate, and optionally _validate_feature_states.
• Update load_spec, load_spec_with_composition, and load_spec_with_environment to only load/merge YAML dictionaries and then delegate validation to _validate_spec_data.
• Preserve existing SpecLoadError messages for schema-version rejection and other validation errors to keep test expectations stable.
• Add focused tests in tests/test_config.py covering schema-version rejection for all three loader paths (load_spec, load_spec_with_composition, and load_spec_with_environment).

Testing

• python -m py_compile netengine/spec/loader.py ran and succeeded to verify syntax correctness of the modified module.
• pytest tests/test_config.py was attempted but could not run to completion due to an environment import error (ModuleNotFoundError: No module named 'logs') reported by test collection, so the new tests were added but not executed in this environment.

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Codex Task