LS: Fix hangs while spawning diagnostics tasks

[mkaput]

Stack:

• LS: Make FileDiagnostics use URLs exclusively to survive db swaps #6681
• LS: Improve tracing spans in diagnostics refreshing #6680
• LS: Fix hangs while spawning diagnostics tasks #6679 ⬅
• LS: Implement multithreaded diagnostics refreshing #6658

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[reviewable-StarkWare]

This change is 

[piotmag769]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Arcticae, @Draggu, and @orizi)

[piotmag769]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Arcticae, @Draggu, and @orizi)

[Draggu]

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Arcticae and @orizi)

[Arcticae]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @orizi)