python: validate offsets to prevent OOB reads in _re2

[Naveed8951]

Description

This PR fixes a memory-safety issue in the RE2 Python extension (_re2)
where user-controlled offsets could cause out-of-bounds reads and crash
the hosting process.

The helper functions that convert between character offsets and byte
offsets performed unchecked pointer arithmetic
(text.data() + pos / text.data() + endpos) and then dereferenced the
result. If a caller supplied a negative or out-of-range offset, the code
could read outside the buffer bounds, resulting in a native crash.

Changes

• Added strict validation in the Python binding helpers:
  • Require a 1D, C-contiguous, byte-addressed buffer (itemsize == 1)
  • Validate pos, endpos, and len ranges before pointer arithmetic
  • Fail safely by raising the module’s Python exception instead of
    performing unsafe memory access
• Added regression tests covering invalid offsets

Impact

• Prevents out-of-bounds reads at the Python/C++ boundary
• Converts a crash condition into safe, predictable Python exceptions
• No behavior change for valid inputs

Testing

• Added unit tests asserting that invalid offsets raise exceptions:
  • negative pos
  • negative len
  • pos > endpos
  • endpos out of range