Fix ends_with returning False for an empty suffix

[gaoflow]

What

ends_with returns False for an empty suffix, contradicting Python's
str.endswith('') / bytes.endswith(b'') and its own starts_with sibling.

'Rule Engine'.ends_with('')     # False — should be True
'Rule Engine'.starts_with('')   # True  (sibling is correct)
[1, 2].ends_with([])            # False — should be True

Why

_value_ends_with slices from the back:

return value[-len(suffix):] == suffix

For an empty suffix, -len(suffix) is -0 == 0, so the slice is value[0:]
— the whole value — and whole == empty is False. The sibling
_value_starts_with slices from the front (value[:len(prefix)]), which for
an empty prefix is value[:0] = an empty slice, correctly yielding True.
The two handlers disagree on the empty case even though every non-empty case
matches.

Fix

Index from the front so an empty suffix yields an empty slice:

return value[len(value) - len(suffix):] == suffix

Equivalent to the old expression for every non-empty suffix; correct for the
empty one.

Tests

Added test_ast_expression_value_methods_empty_affix, exercising STRING,
BYTES and ARRAY through both ends_with and starts_with, asserting that an
empty affix and the whole value both match. It fails before the change
(ends_with('') should be True) and passes after. Full suite: Ran 455 tests ... OK (skipped=31); mypy clean on the changed file.

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This pull request was prepared with the assistance of AI, under my direction and review.