fix OOB slab panic when creating runnables on a did_save event

[JoshuaBatty]

Description

I noticed that the language server would crash when compilation was triggered from a did save event as we were trying to create runnables from the Engines that had GC applied to it. We now check to ensure that the correct Engines is accessed to avoid a panic when creating runnables.

After fixing this bug, I noticed that optimised builds was not returning diagnostics. I've opened #6576 to work on this and have temporarily disabled the optimisation so diagnostics continue to work.

Checklist

• [x] I have linked to any relevant issues.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • [ ] If my change requires substantial documentation changes, I have requested support from the DevRel team
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #6575 will degrade performances by 18.06%

_{Comparing josh/panic_on_save (8cc5062) with master (28c8ae4)}

Summary

❌ 1 regressions ✅ 21 untouched benchmarks

:warning: _Please fix the performance issues or acknowledge them on CodSpeed._

Benchmarks breakdown

	Benchmark	master	josh/panic_on_save	Change
❌	document_symbol	4.4 ms	5.4 ms	-18.06%