test(lsp): extract buf/util parts from lsp_spec.lua #39149

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.

test/functional/treesitter/util_spec.lua

@@ -0,0 +1,50 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+
+local clear = n.clear
+local insert = n.insert
+local eq = t.eq
+local exec_lua = n.exec_lua
+
+before_each(clear)
+
+describe('treesitter utils', function()
+  it('can find an ancestor', function()
+    insert([[
+      int main() {
+        int x = 3;
+      }]])
+
+    exec_lua(function()
+      local parser = vim.treesitter.get_parser(0, 'c')
+      local tree = parser:parse()[1]
+      local root = tree:root()
+      _G.ancestor = assert(root:child(0))
+      _G.child = assert(_G.ancestor:named_child(1))
+      _G.child_sibling = assert(_G.ancestor:named_child(2))
+      _G.grandchild = assert(_G.child:named_child(0))
+    end)
+
+    eq(true, exec_lua('return vim.treesitter.is_ancestor(_G.ancestor, _G.child)'))
+    eq(true, exec_lua('return vim.treesitter.is_ancestor(_G.ancestor, _G.grandchild)'))
+    eq(false, exec_lua('return vim.treesitter.is_ancestor(_G.child, _G.ancestor)'))
+    eq(false, exec_lua('return vim.treesitter.is_ancestor(_G.child, _G.child_sibling)'))
+  end)
+
+  it('can detect if a position is contained in a node', function()
+    exec_lua(function()
+      _G.node = {
+        range = function()
+          return 0, 4, 0, 8
+        end,
+      }
+    end)
+
+    eq(false, exec_lua('return vim.treesitter.is_in_node_range(node, 0, 3)'))
+    for i = 4, 7 do
+      eq(true, exec_lua('return vim.treesitter.is_in_node_range(node, 0, ...)', i))
+    end
+    -- End column exclusive
+    eq(false, exec_lua('return vim.treesitter.is_in_node_range(node, 0, 8)'))
+  end)
+end)