fix: preserve explicit false SSL options

[fengjikui]

Fixes #2557.

Summary

• preserve explicit falsy SSL options instead of treating them as missing
• keep client_args={"verify": False} / async_client_args={"verify": False} as False for httpx
• keep async_client_args={"ssl": False} as False for aiohttp and websocket paths
• add regression coverage for the explicit False cases

Root cause

The SSL helpers used truthiness checks such as if not ctx and if not args.get(...). That makes an explicit False indistinguishable from an unset option, so the SDK replaced the user's disabled-verification setting with a default ssl.SSLContext.

Validation

• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy .venv/bin/python -m pytest google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_implicit_initialization google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_explicit_initialization_same_args google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_explicit_initialization_separate_args google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_explicit_initialization_sync_args google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_explicit_initialization_async_args google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_preserves_explicit_false_httpx_verify google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_preserves_explicit_false_aiohttp_ssl google/genai/tests/client/test_client_initialization.py::test_client_ssl_context_preserves_explicit_false_websocket_ssl -q (10 passed)
• env -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY -u http_proxy -u https_proxy -u all_proxy .venv/bin/python -m pytest google/genai/tests/client/test_client_initialization.py -q (90 passed)
• git diff --check

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