fix(lsp): improper diagnostic end_col computation

**Problem:** For multiline diagnostics, the end column was improperly
calculated by checking the byte index of the character position on the
*start* line.

**Solution:** Calculate the byte index for end_col using the *end* line.

runtime/lua/vim/lsp/diagnostic.lua

@@ -100,12 +100,16 @@ local function diagnostic_lsp_to_vim(diagnostics, bufnr, client_id)
       message = diagnostic.message.value
     end
     local line = buf_lines and buf_lines[start.line + 1] or ''
+    local end_line = line
+    if _end.line > start.line then
+      end_line = buf_lines and buf_lines[_end.line + 1] or ''
+    end
     --- @type vim.Diagnostic
     return {
       lnum = start.line,
       col = vim.str_byteindex(line, position_encoding, start.character, false),
       end_lnum = _end.line,
-      end_col = vim.str_byteindex(line, position_encoding, _end.character, false),
+      end_col = vim.str_byteindex(end_line, position_encoding, _end.character, false),
       severity = severity_lsp_to_vim(diagnostic.severity),
       message = message,
       source = diagnostic.source,

test/functional/plugin/lsp/diagnostic_spec.lua

@@ -108,7 +108,7 @@ describe('vim.lsp.diagnostic', function()
 
     exec_lua(function()
       diagnostic_bufnr = vim.uri_to_bufnr(fake_uri)
-      local lines = { '1st line of text', '2nd line of text', 'wow', 'cool', 'more', 'lines' }
+      local lines = { '1st line', '2nd line of text', 'wow', 'cool', 'more', 'lines' }
       vim.fn.bufload(diagnostic_bufnr)
       vim.api.nvim_buf_set_lines(diagnostic_bufnr, 0, 1, false, lines)
       vim.api.nvim_win_set_buf(0, diagnostic_bufnr)
@@ -285,6 +285,36 @@ describe('vim.lsp.diagnostic', function()
       eq('Pull Diagnostic', diags[1].message)
     end)
 
+    it('handles multiline diagnostic ranges #33782', function()
+      local diags = exec_lua(function()
+        vim.lsp.diagnostic.on_diagnostic(nil, {
+          kind = 'full',
+          items = {
+            _G.make_error('Pull Diagnostic', 0, 6, 1, 10),
+          },
+        }, {
+          params = {
+            textDocument = { uri = fake_uri },
+          },
+          uri = fake_uri,
+          client_id = client_id,
+          bufnr = diagnostic_bufnr,
+        }, {})
+
+        return vim.diagnostic.get(diagnostic_bufnr)
+      end)
+      local lines = exec_lua(function()
+        return vim.api.nvim_buf_get_lines(diagnostic_bufnr, 0, -1, false)
+      end)
+      -- This test case must be run over a multiline diagnostic in which the start line is shorter
+      -- than the end line, and the end_col exceeds the start line's length.
+      eq(#lines[1], 8)
+      eq(#lines[2], 16)
+      eq(1, #diags)
+      eq(6, diags[1].col)
+      eq(10, diags[1].end_col)
+    end)
+
     it('severity defaults to error if missing', function()
       ---@type vim.Diagnostic[]
       local diagnostics = exec_lua(function()