fix(streamlit): don't require tornado on streamlit >= 1.57

[MohammadHijjawi97]

Fixes #743

Streamlit 1.57 moved its server to Starlette/uvicorn and dropped its Tornado dependency. Main already handles part of #743: it has a fallback for make_url_path_regex and registers a Starlette route. But pygwalker/communications/streamlit_comm.py still imports tornado unconditionally. With a current Streamlit (e.g. 1.64) and no tornado installed, from pygwalker.api.streamlit import StreamlitRenderer still fails with ModuleNotFoundError: No module named 'tornado'.

Changes:

• Import tornado optionally, and define PygwalkerHandler only when tornado is available.
• Without tornado, _register_tornado_handler() returns False, so _hack_streamlit_server_impl falls through to the existing Starlette route registration.
• Regression test: with tornado blocked, streamlit_comm and pygwalker.api.streamlit import cleanly and the route regex helper produces the expected pattern.

Testing:

• pytest tests/test_integration_apis.py: 30 passed with streamlit 1.56.0 (tornado installed) and with streamlit 1.64.0 (no tornado). The new test fails without the fix.
• Ran streamlit run with StreamlitRenderer(df).explorer() on 1.56 and on 1.64. The explorer loads and POST /_stcore/_pygwalker/comm/<gid> returns a successful response in both.
• ruff check and ruff format --check are clean on the changed files.

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