test(lsp): extract buf/util parts from lsp_spec.lua
Problem:
`test/functional/plugin/lsp_spec.lua` had grown into a large catch-all file that mixed core LSP client lifecycle coverage, `vim.lsp.buf.*` behavior, and `vim.lsp.util.*` behavior in one place.
Solution:
Split the large tests into more focused test files without changing test coverage or intended behavior.
After this change, `lsp_spec.lua` is more focused on core LSP client/config/dynamic-registration behavior.
Co-authored-by: Yi Ming <ofseed@foxmail.com>
