Make LSP completions resolve capabilities more spec-compliant #5497

Job	Run time
test (ubuntu-latest, 27.2, false)	2m 12s
test (ubuntu-latest, 28.2, false)	2m 38s
test (ubuntu-latest, 29.4, false)	3m 27s
test (macos-latest, 28.2, false)	4m 49s
test (macos-latest, 29.4, false)	4m 36s
test (windows-latest, 27.2, false)	4m 37s
test (windows-latest, 28.2, false)	5m 21s
test (windows-latest, 29.4, false)	3m 33s
test (ubuntu-latest, snapshot, true)	1m 20s
test (macos-latest, snapshot, true)	2m 52s
test (windows-latest, snapshot, true)	5m 25s
	40m 50s

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