Summary

• Clear the JavaScript FSEvent wrapper from FSWatcher._handle when a
  missing path is tolerated with throwIfNoEntry: false.
• Update test/parallel/test-fs-watch-enoent.js to exercise
  FSWatcher.close() after giving the native close callback an opportunity
  to run on Linux.
• Add test/parallel/test-watch-mode-missing-env-file-signal.mjs to verify
  clean SIGINT and SIGTERM exits when an optional env file is missing.

Details

On Linux, uv_fs_event_start() returns UV_ENOENT for a missing path.
FSEventWrap::Start() starts closing the native handle before returning that
error. When throwIfNoEntry is false, FSWatcher._handle previously kept a
reference to the JavaScript FSEvent wrapper. A later FSWatcher.close()
accessed that wrapper's initialized getter after the native FSEventWrap
had been detached, causing the FSEventWrap::GetInitialized() null-wrapper
assertion.

Set FSWatcher._handle to null in the tolerated UV_ENOENT branch. This is
only a JavaScript lifecycle-state transition; it does not close the native
handle a second time or emit a close event.

The Linux regression test uses common.platformTimeout(10), a non-zero
delayed timer, before calling close(). A next tick or immediate can run
before the native close callback and therefore would not reliably exercise
the detached-wrapper state.

The watch-mode regression test starts separate processes for SIGINT and
SIGTERM, waits for watch-mode readiness, and checks for a zero exit code, no
terminating signal, and no native assertion output. It also has readiness and
shutdown timeouts and always cleans up a child on failure.

Testing

• Official Docker baselines reproduced the native assertion with
  node:24.16.0-slim, node:24.18.0-slim, and node:26.5.0-slim.
• The pre-fix baseline was also reproduced locally at upstream/HEAD
41525ab9fcb92d6f487047b024be29a1a96d639c
  with exit code 134.
• The patch was rebased onto upstream/HEAD
012ecf51d39d784f1c217f941122717688c31add.
• test/parallel/test-fs-watch-enoent.js and
  test/parallel/test-watch-mode-missing-env-file-signal.mjs passed
  individually and for 20 repetitions each.
• The watch-mode regression test passed for 20 repetitions in an Ubuntu 26.04
  container using the patched build.
• Existing-path, default throwIfNoEntry, immediate-close, repeated-close,
  ref()/unref(), AbortSignal, and standalone watch-mode controls passed.
• A 2,000-watcher stress check observed 2,000 FSEVENTWRAP initializations and
  2,000 destroys.
• make lint passed.
• git diff --check passed.
• make -j4 test passed on 012ecf51d39d784f1c217f941122717688c31add
  in a clean test environment: 5,790 passed, 0 failed.

Fixes: #64819
Refs: #61870
Refs: #20985