Make tuple index constexpr-safe; fix Python Protocol import; tighten XLA typings - #328

Motivation

• Ensure compile-time computation of a type's index in a tuple is robust and portable across compilers by replacing the previous fold-expression approach.
• Make the Protocol import deterministic by checking the running Python version instead of relying on a try/except fallback only.
• Improve type clarity in XLA helper functions to better reflect arbitrary-length spec tuples.

Description

• Rewrote Index<T, std::tuple<...>> in envpool/core/tuple_utils.h to use a constexpr std::array<bool, ...> scan to compute kValue, added #include <array>, and moved the static_assert to validate the found index.
• Kept TupleFromVectorImpl overloads intact but consolidated the constexpr indexing logic for clearer behavior and error messages.
• In envpool/python/protocol.py added import sys and replaced the try/except import of Protocol with a version check that uses typing.Protocol for Python >= 3.8 and typing_extensions.Protocol otherwise.
• In envpool/python/xla_template.py updated the type annotation for specs to Tuple[Tuple[Any, ...], Tuple[Any, ...]] to reflect variable-length inner tuples and adjusted formatting accordingly.

Testing

• Built the C++ library and ran the project's automated unit test suite, which completed successfully.
• Ran Python static type checks and linters on the modified files, which passed without errors.

Codex Task