When a packet with ALLOW_SHORT receives an undersized buffer, items
that are completely outside the buffer bounds now return nil/None
instead of 0. Previously, undersized buffers were padded with zeros
which caused out-of-bounds items to return 0.

Changes:
- Structure no longer resizes buffer when short_buffer_allowed is true
- BinaryAccessor.read_item checks bounds before reading and returns
  nil/None for items outside the buffer
- Added item_within_buffer_bounds? method to check if item fits in buffer
- Updated comments in accessor classes to reflect new behavior

Fixes #2463

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Co-Authored-By: Claude Opus 4.5 <[email protected]>

Address PR review comment by moving the bounds checking logic into
the existing low-level read functions instead of having a separate
item_within_buffer_bounds? check. This eliminates duplicate bounds
checking and keeps the check in the C code for better performance.

Changes:
- C code returns nil instead of raising on bounds error
- Ruby/Python read methods return nil on bounds failure
- Added bounds check to read_array to return nil for undersized buffers
- Removed item_within_buffer_bounds method and its call from read_item
- Removed unnecessary resize_buffer call in structure files
- Updated tests to expect nil instead of errors for out-of-bounds reads

Co-Authored-By: Claude Opus 4.5 <[email protected]>

…ackets

Add nil guards to prevent errors when items return nil for being outside
buffer bounds in undersized packets:

- Add nil guard in check_limits to skip limits checking for nil values
- Add nil guard in apply_format_string_and_units to return nil early
- Add nil guard in polynomial_conversion (Ruby, C extension, and Python)
- Add nil guard in segmented_polynomial_conversion (Ruby and Python)

Update tests to use properly sized buffers or expect nil for out-of-bounds
items.

Co-Authored-By: Claude Opus 4.5 <[email protected]>