fix: respect cody.useContext for context fetching

[abeatrix]

CLOSE #2615

fix: add config to control context fetching strategy

This adds the exisiting 'cody.useContext' config to SimpleChatPanelProvider that allows selecting between 'embeddings', 'keyword', and 'none' strategies for context fetching.

• 'embeddings' will use embeddings-based indexing if available, falling back to keyword search (symf).
• 'keyword' will only use keyword search-based (symf) context fetching.
• 'none' will limit context to the user's current open file.

The default is now 'embeddings'

Test plan

1. Start build from this branch
2. Set cody.useContext to search to confirm context from embeddings are not added, and context from symf are added (if any)
3. Set cody.useContext to embeddings to confirm context from embeddings are added (if any)
4. Set cody.useContext to none to confirm only context from open file (if any) is added