Hardening: extract operator prefixes from constraint values

[irfan-evoila]

Refactored the monolithic LabelProcessor (774 lines) into a clean linear pipeline with clear separation of concerns:
Parse -> Normalize -> Enhance -> Validate -> Serialize.

Each step is now handled by a focused class (LabelParser, LabelNormalizer, LabelEnhancer), resulting in a more robust and maintainable architecture for future changes

Constraint values can encode operators as prefixes (e.g. !~^kube-.*).

Previously, these were treated as raw values and wrapped with the default =~ operator,
producing incorrect expressions like k8s_namespace_name="!^kube-.*".

Now LabelEnhancer extracts !, =, and != prefixes from constraint values and uses them as the actual operator.