fix(lsp): fix off-by-one error for omnifunc word boundary

Fixes https://github.com/neovim/neovim/issues/25177

I initially wanted to split this into a refactor commit to make it more
testable, but it appears that already accidentally fixed the issue by
normalizing lnum/col to 0-indexing

test/functional/plugin/lsp/completion_spec.lua

@@ -0,0 +1,183 @@
+---@diagnostic disable: no-unknown
+local helpers = require('test.functional.helpers')(after_each)
+local eq = helpers.eq
+local exec_lua = helpers.exec_lua
+
+
+--- Convert completion results.
+---
+---@param line string line contents. Mark cursor position with `|`
+---@param candidates lsp.CompletionList|lsp.CompletionItem[]
+---@param lnum? integer 0-based, defaults to 0
+---@return {items: table[], server_start_boundary: integer?}
+local function complete(line, candidates, lnum)
+  lnum = lnum or 0
+  local cursor_col = line:find("|")
+  line = line:gsub("|", "")
+  return exec_lua([[
+    local line, cursor_col, lnum, result = ...
+    local line_to_cursor = line:sub(1, cursor_col)
+    local client_start_boundary = vim.fn.match(line_to_cursor, '\\k*$')
+    local items, server_start_boundary = require("vim.lsp._completion")._convert_results(
+      line,
+      lnum,
+      client_start_boundary,
+      nil,
+      result,
+      "utf-16"
+    )
+    return {
+      items = items,
+      server_start_boundary = server_start_boundary
+    }
+  ]], line, cursor_col, lnum, candidates)
+end
+
+
+describe("vim.lsp._completion", function()
+  before_each(helpers.clear)
+
+  -- https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion
+  it('prefers textEdit over label as word', function()
+    local range0 = {
+      start = { line = 0, character = 0 },
+      ["end"] = { line = 0, character = 0 },
+    }
+    local completion_list = {
+      -- resolves into label
+      { label = 'foobar', sortText = 'a', documentation = 'documentation' },
+      {
+        label = 'foobar',
+        sortText = 'b',
+        documentation = { value = 'documentation' },
+      },
+      -- resolves into insertText
+      { label = 'foocar', sortText = 'c', insertText = 'foobar' },
+      { label = 'foocar', sortText = 'd', insertText = 'foobar' },
+      -- resolves into textEdit.newText
+      { label = 'foocar', sortText = 'e', insertText = 'foodar', textEdit = { newText = 'foobar', range = range0 } },
+      { label = 'foocar', sortText = 'f', textEdit = { newText = 'foobar', range = range0 } },
+      -- real-world snippet text
+      {
+        label = 'foocar',
+        sortText = 'g',
+        insertText = 'foodar',
+        insertTextFormat = 2,
+        textEdit = { newText = 'foobar(${1:place holder}, ${2:more ...holder{\\}})', range = range0 },
+      },
+      {
+        label = 'foocar',
+        sortText = 'h',
+        insertText = 'foodar(${1:var1} typ1, ${2:var2} *typ2) {$0\\}',
+        insertTextFormat = 2,
+      },
+      -- nested snippet tokens
+      {
+        label = 'foocar',
+        sortText = 'i',
+        insertText = 'foodar(${1:${2|typ1,typ2|}}) {$0\\}',
+        insertTextFormat = 2,
+      },
+      -- braced tabstop
+      { label = 'foocar', sortText = 'j', insertText = 'foodar()${0}', insertTextFormat = 2},
+      -- plain text
+      {
+        label = 'foocar',
+        sortText = 'k',
+        insertText = 'foodar(${1:var1})',
+        insertTextFormat = 1,
+      },
+    }
+    local expected = {
+      {
+        abbr = 'foobar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foobar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foobar',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foobar(place holder, more ...holder{})',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foodar(var1 typ1, var2 *typ2) {}',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foodar(typ1) {}',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foodar()',
+      },
+      {
+        abbr = 'foocar',
+        word = 'foodar(${1:var1})',
+      },
+    }
+    local result = complete('|', completion_list)
+    result = vim.tbl_map(function(x)
+      return {
+        abbr = x.abbr,
+        word = x.word
+      }
+    end, result.items)
+    eq(expected, result)
+  end)
+  it("uses correct start boundary", function()
+    local completion_list = {
+      isIncomplete = false,
+      items = {
+        {
+          filterText = "this_thread",
+          insertText = "this_thread",
+          insertTextFormat = 1,
+          kind = 9,
+          label = " this_thread",
+          score = 1.3205767869949,
+          sortText = "4056f757this_thread",
+          textEdit = {
+            newText = "this_thread",
+            range = {
+              start = { line = 0, character = 7 },
+              ["end"] = { line = 0, character = 11 },
+            },
+          }
+        },
+      }
+    }
+    local expected = {
+      abbr = ' this_thread',
+      dup = 1,
+      empty = 1,
+      icase = 1,
+      kind = 'Module',
+      menu = '',
+      word = 'this_thread',
+    }
+    local result = complete("  std::this|", completion_list)
+    eq(7, result.server_start_boundary)
+    local item = result.items[1]
+    item.user_data = nil
+    eq(expected, item)
+  end)
+end)