fix: enable suggestions cache by default

Description

Enables the LS code suggestions cache by default for users of Workflow.

The default setting still may be overridden by applying the appropriate user/workspace/folder setting (gitlab.aiAssistedCodeSuggestions.suggestionsCache.enabled: true).

Related Issues

Resolves #1155 (closed)

How has this been tested?

1. Clear your user/workspace/folder settings of anything containing gitlab.aiAssistedCodeSuggestions.suggestionsCache
2. Run the extension host.
3. Write some code that triggers an ai-assisted code suggestion.
4. Add some characters to trigger a new suggestion.
5. Delete characters back to the original suggestion.
6. Observe that the suggestion returns instantly, proving it's coming from the cache.

Screenshots (if appropriate):

before	after
cache_default	cache_enabled

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap