Add IntelliSense support for HelmRelease values field

[yebyen]

Summary

This PR adds IntelliSense (auto-completion, hover documentation, and validation) for the values field in Flux HelmRelease resources. When editing HelmRelease YAML files, users now get intelligent suggestions based on the Helm chart's values.schema.json.

Changes

• Add src/language/ module with HelmRelease detection and parsing
• Implement schema fetching from HelmRepository sources with caching
• Register VSCode completion provider for values field auto-completion
• Register hover provider for property documentation
• Register diagnostic provider for values validation
• Add unit tests for HelmRelease detection
• Update README with feature documentation

How it works

1. Extension detects HelmRelease resources in YAML files
2. Parses chart reference (name, version, sourceRef)
3. Fetches values.schema.json from the Helm chart archive
4. Provides completions, hover docs, and validation based on schema

Testing

• Unit tests for HelmRelease detection logic
• Manual testing with Helm charts that have schemas (e.g., Traefik)
• Graceful degradation when schema is unavailable

Notes

• Only HelmRepository sources supported initially (GitRepository/OCI planned for future)
• Schemas are cached to disk with 24-hour TTL for offline use
• Charts without values.schema.json silently skip IntelliSense features

Fixes weaveworks#512

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🔗 Open in Helix

📋 Spec:

• Requirements
• Design
• Tasks

🚀 Built with Helix

[mewzherder]

See review #1 notes here: https://github.com/yebyen/vscode-gitops-tools/blob/helix-specs/design/tasks/000379_test-the-code-in-this-pr/pr_comment.md