What this PR does

Fixes a fatal error: concurrent map writes crash in workflow execution (reported in #2713).

Root cause

WorkflowError values carry an error level. The level was stored inside the
underlying errorx.StatusError's Extra() map, and ChangeErrLevel mutated
that map in place:

func (w *wfErr) ChangeErrLevel(newLevel ErrorLevel) WorkflowError {
    w.StatusError.Extra()["level"] = string(newLevel) // in-place write
    return w
}

Several WorkflowError values are package-level singletons that are shared by
every goroutine:

var CancelErr = newCancel()
var NodeTimeoutErr = newNodeTimeout()
var WorkflowTimeoutErr = newWorkflowTimeout()

When a node fails, (*nodeRunner).onError assigns one of these shared
singletons (e.g. sErr = vo.NodeTimeoutErr on context.DeadlineExceeded) and
then calls sErr.ChangeErrLevel(vo.LevelWarn). Under concurrent workflow
execution — e.g. many requests whose HTTP nodes time out at the same time —
multiple goroutines write to the same shared map simultaneously, which Go
detects as a fatal, unrecoverable concurrent map writes and crashes the
whole process. This exactly matches the stack trace in #2713
(node.go:151 in ChangeErrLevel, node_runner.go:848 in onError).

The fix

• Give wfErr its own level ErrorLevel field instead of storing the level in
  the shared Extra() map.
• ChangeErrLevel now returns a copy of the error with the new level and
  never mutates the receiver, so the shared singletons are never modified.
• Level() prefers the new field and falls back to the previous Extra()
  lookup, so behavior is unchanged for existing callers (event_handle.go).
• Constructors populate the field; the Extra("level", ...) metadata is kept
  for backward compatibility.

After the fix the singletons' Extra() map is only written once at
construction and is read-only afterwards, so concurrent access is safe.

Tests

Added node_concurrency_test.go:

• TestChangeErrLevelIsSafeForConcurrency — 100 goroutines call
  ChangeErrLevel on the shared singletons concurrently. This reproduces the
  crash on the old code and passes on the new code (also clean under -race).
• TestChangeErrLevelDoesNotMutateSingleton — verifies ChangeErrLevel
  returns a copy and leaves the singleton's level untouched.

Fixes #2713