feat(harper-ls): add configurable diagnostic delay

[Dronakurl]

feat(harper-ls): Add configurable diagnostic delay

This PR is now hand written. I took the code generated by vibe and stripped it down. In the end, I basically touched every line.

• Problem: I happily use harper-ls within helix and I found it distracting that the diagnostics pop up immediately while typing.
• Solution: I introduced a new config option for a delay in the diagnostics. While typing, no new diagnostics show up. Only if I stop typing for a (configurable) time, new diagnostics pop up.
• Code actions: I removed the code that immediately triggered code actions for replacing words. It was too complicated. When adding a word to a dictionary, immediate change of the diagnostics is supported.
• Technical remark:
  • The idea is to spawn a pending diagnostic task on each change of the file. If you type, the tasks are aborted and restarted. Only when after the specified time, no changes were made, the diagnostic task goes through
  • In order to spawn the diagnostic in a separate thread, I needed to make the Backend Clone by wrapping everything in Arc.
  • When the delay in seconds is not defined (defaults to 0), none of this is done to save resources for everyone who is not using the feature.

This PR was initially created by vibe coding agents and I changed the AI-written explanation here. Just if you wonder about the comments below.

[elijah-potter]

It looks like you've broken some VS Code tests. You might want to try extending their timeouts to account for the default delay. It might be an easy fix.

[elijah-potter]

@Dronakurl, have you read our agent policy?

[Dronakurl]

Thank you for pointing that out to me.

I have read it now. :-)

So here is the brief, non LLM-generated description of the state of the PR:

• This PR was generated by an LLM agent (I use mistral vibe).
• Problem: I happily use harper-ls within helix and I found it distracting that the diagnostics pop up immediately while typing.
• Solution:
  • I introduced a new config option for a delay in the diagnostics. While typing, no new diagnostics show up. Only if I stop typing for a (configurable) time, new diagnostics pop up.
  • Initially, there was a problem with code actions not being applied immediately, because the new delay interfered. That was solved, too.
• Testing: I have used it for 2 weeks and did not see an issue. What I honestly did not test (apart from the workflow tests) is if there are any side effects on vs code, etc.

I am not an experienced rust programmer, to be honest, but I looked at the code myself and found it reasonable (unlike this other language feature, which should not have been marked "ready for review" in the first place).

Could you please have a brief look at the code and tell me the first thing you find problematic? (The first thing in the changed files is this "pr_drafts" folder, which should not be in there, so maybe the second thing. :-))
I think, I can work with that to revise the PR so that it is acceptable.

[Dronakurl]

Updated the PR description, since it was only AI stuff, so now I would be grateful for a review on this PR. @elijah-potter