Add and implement manual_render

[pienter-tech]

Virtual Text Manual Rendering Enhancement

Background

Currently, when using virtual text in manual mode, completions are only fetched when explicitly requested. This leads to:

1. Slower response times due to on-demand fetching
2. Potentially lower quality suggestions due to limited context from sparse fetching

Changes

Introduced a new configuration option virtual_text.manual_render that:

• When enabled (and manual = false), completions are fetched continuously in the background
• Virtual text suggestions are only rendered when cycle_or_complete is triggered
• Combines the benefits of background context building with manual rendering control

Benefits

• Faster completion display when requested
• Better suggestion quality due to continuous context building
• Maintains clean UI by only showing suggestions on demand

Any feedback is welcome.