refactor(sdk-python): hoist HTTPX network error names in exception converter

[Gujiassh]

Summary

• move the static HTTPX network error names from _is_httpx_network_error into a module-level constant
• reuse that constant for membership checks instead of constructing a tuple on every invocation
• keep exception mapping behavior unchanged

Why

_is_httpx_network_error sits on the exception conversion path and can be called repeatedly under transient/network-failure conditions. Hoisting the constant removes repeated tuple construction and centralizes the canonical list of supported HTTPX network error names.

Testing

• python3 -m py_compile sdks/sandbox/python/src/opensandbox/adapters/converter/exception_converter.py
• python3 -m pytest tests/test_converters_and_error_handling.py -q

Compatibility

• no API or behavior changes
• same recognized error names, only storage location changed

[CLAassistant]

All committers have signed the CLA.

[Gujiassh]

Maintainer note: this change stays intentionally behavior-preserving and scoped to a single refactor in the exception conversion path.\n\nIf useful, I can follow up with a separate PR that switches from string-name matching to direct exception-class checks and includes a small benchmark comparison.

[Pangjiping]

LGTM, efficient code refactoring! Please sign the CLA and @ninan-nn will review those changes. 😊

[Gujiassh]

Thanks again — the CLA is signed now and the checks are all green on my side. @ninan-nn, when you have a moment, could you take a look?

[ninan-nn]

Duplicated with #376