Remove deprecated Tornado current-loop APIs in WebGL server thread

[Copilot]

This change removes Tornado deprecation warnings emitted by cortex/webgl/serve.py when running WebGL/headless tests. WebApp.run() previously used deprecated current-loop mutation methods (clear_current / make_current).

• Runtime loop initialization

  • WebApp.run() now gets the thread-local loop via tornado.ioloop.IOLoop.current() instead of constructing an IOLoop and mutating global current-loop state.
  • Existing server startup flow remains the same (Application → HTTPServer → listen → start).

• Focused regression coverage

  • Added cortex/tests/test_webgl_serve.py with a targeted test that monkeypatches IOLoop.current() and asserts WebApp uses that loop for startup.

def run(self):
    ioloop: tornado.ioloop.IOLoop = tornado.ioloop.IOLoop.current()
    self.ioloop = ioloop
    application = tornado.web.Application(self.handlers, gzip=True)
    ...
    self.server.listen(self.port)
    ioloop.start()

Original prompt

Fix these two tornado warnings in cortex/webgl/serve.py:

cortex/tests/test_export.py: 2 warnings
cortex/tests/test_headless.py: 3 warnings
cortex/tests/test_webgl_headless.py: 23 warnings
  /home/runner/work/pycortex/pycortex/cortex/webgl/serve.py:327: DeprecationWarning: clear_current is deprecated
    ioloop.clear_current()

cortex/tests/test_export.py: 2 warnings
cortex/tests/test_headless.py: 3 warnings
cortex/tests/test_webgl_headless.py: 23 warnings
  /home/runner/work/pycortex/pycortex/cortex/webgl/serve.py:328: DeprecationWarning: make_current is deprecated; start the event loop first
    ioloop.make_current()

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