Make LSP completions resolve capabilities more spec-compliant #5479

Job	Run time
test (ubuntu-latest, 27.2, false)	2m 10s
test (ubuntu-latest, 28.2, false)	2m 31s
test (ubuntu-latest, 29.4, false)	2m 26s
test (macos-latest, 28.2, false)	4m 25s
test (macos-latest, 29.4, false)	4m 10s
test (windows-latest, 27.2, false)	4m 19s
test (windows-latest, 28.2, false)	3m 23s
test (windows-latest, 29.4, false)	2m 56s
test (ubuntu-latest, snapshot, true)	1m 24s
test (macos-latest, snapshot, true)	2m 44s
test (windows-latest, snapshot, true)	4m 46s
	35m 14s

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