python: reject non-contiguous buffers

[mraimadeddinehouas-glitch]

The Python binding requests a full buffer view but previously treated every buffer as a contiguous one-dimensional byte string. This is unsafe for non-contiguous, multi-dimensional, or wider-item buffers: the pointer arithmetic and byte length do not describe the logical view.

This change validates the buffer layout before constructing an absl::string_view. Unsupported layouts are rejected unless the buffer is one-dimensional, byte-sized, and has a unit stride.

A regression test covers reversed and strided memoryviews for both pattern and input paths.

Validation:

• RE2 C++ CMake suite: 20/20 tests passed.
• • Python binding suite: 244 tests passed.
• • Independent comparison: the parent revision accepted strided, reversed, two-dimensional, and wide-item buffers; this revision rejects all four.
• • No upstream source files outside the Python binding and its tests are changed.

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[mraimadeddinehouas-glitch]

Hello RE2 maintainers — when convenient, could you please review this PR? The patch is narrowly scoped to the Python binding, the C++ and Python test suites pass, the Google CLA check passes, and the commit is verified. The PR is ready for review; thank you.