Closes #36981 

- Add completion text and position caching to reduce redundant API calls
- Only trigger new completion requests on text changes, not cursor
movement
- Validate cursor position to ensure completions show at correct
location
- Improve end-of-line range calculation for more accurate deletions
- Extract reset_completion_cache helper for cleaner code organization
- Update completion diff algorithm documentation for clarity

Edit: Sorry this is the 2nd PR, I forgot that the forks history was
messy; I cherrypicked and cleaned it properly with this PR

Release Notes:

- supermaven: Improved caching of predictions
- supermaven: Fixed an issue where changing cursor position would
incorrectly trigger new completions