refactor(vulnfeeds): utilise a strategy pattern on a CNA basis to reduce carveout logic

[jess-lowe]

Refactors CVE5 version extraction from nested fallback branches into a modular, priority-based strategy pipeline. This will be so much easier to extend as we get better insights on CNA quirks.

Highlights

• Strategy Pipeline: Introduced VersionStrategy with configurable priority levels (PriorityFirst, PriorityStandard, PriorityLastResort) and CNA-specific presets (Default, GitHub, MITRE, Linux).
• Extracted Strategies: Isolated parsing for standard bounds, split/unspecified ranges, changes blocks, string range expressions, git commit hashes, CPE applicability, and single-version heuristics.
• Direct Git Ranges: Bypass tag-to-commit normalization when ranges already contain commit SHAs via IsDirectGitRange.
• Early Exit: If repos are unavailable, cleanly record unresolved ranges to database_specific.unresolved_ranges without running redundant fallbacks.
• Tests: Added comprehensive unit tests for each strategy, priority ordering, and split/git ranges.