Promote CLI override parsing to config loader

[aidankhogg]

Motivation

• Factor the CLI --set dotted key=value override parsing into a reusable config helper so other code can reuse the logic and keep parsing behavior consistent.
• Preserve existing CLI semantics: require key=value, reject empty dotted path components, parse values with YAML semantics, and disallow nesting under scalar values.

Description

• Add ConfigOverrideError and parse_dotted_overrides(values: Iterable[str]) -> dict[str, Any] to netengine/config/loader.py, implementing YAML safe_load value parsing, empty-part validation, and scalar-vs-object conflict detection.
• Replace the inline parser in netengine/cli/main.py with a small shim _parse_set_overrides that delegates to parse_dotted_overrides and converts ConfigOverrideError into click.BadParameter with the --set hint.
• Add unit tests in tests/test_config.py (TestParseDottedOverrides) covering nested overrides, YAML scalar/list parsing, malformed inputs, and scalar/nested conflicts.
• Remove the now-unused direct YAML parsing code from the CLI module and keep the single source of truth in the config loader.

Testing

• Run static checks with python -m ruff check netengine/config/loader.py netengine/cli/main.py tests/test_config.py which passed.
• Run unit tests with pytest tests/test_config.py tests/test_cli.py -q, which succeeded (all tests passed: 64 passed).

---

Codex Task