language: Defer auto-indent until parsing catches up (#62024)

# Objective

Prevent auto-indent from using stale syntax when text changes during a
background parse.

## Solution

Keep auto-indent pending and parsing active until the current parse
completes.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- Editor: Fixed auto-indent occasionally using stale syntax after rapid
edits.