Preserve omitted slice positions and validate builtin sequence slices

[ak4-sh]

Validate slice values for builtin sequences

Add tests to check for correct slice behaviour

Remove bug status from testcase since behaviour is fixed

Summary

Validate slice components for builtin list and tuple subscripts. Pyrefly now reports errors for non-index slice values like decimal steps, and for literal zero steps. Also preserve omitted slice positions when inferring slice[...] types so start, stop, and step do not shift into the wrong generic slots.

Remove the bug marker from the existing zero-step slice test since the behaviour is now fixed, and add list/tuple coverage for decimal and zero step slices.

Fixes #3768, #3896

Test Plan

• cargo test test_slice_decimal_step
• cargo test test_slice_zero_step

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
- ERROR python/pyspark/pandas/tests/test_typedef.py:190:37-53: Expected a type form, got instance of `slice[Literal['a'], type[float64], Literal['a'] | type[float64]]` [not-a-type]
+ ERROR python/pyspark/pandas/tests/test_typedef.py:190:37-53: Expected a type form, got instance of `slice[Literal['a'], type[float64], None]` [not-a-type]
- ERROR python/pyspark/pandas/tests/test_typedef.py:190:55-62: Expected a type form, got instance of `slice[Literal['b'], type[str], Literal['b'] | type[str]]` [not-a-type]
+ ERROR python/pyspark/pandas/tests/test_typedef.py:190:55-62: Expected a type form, got instance of `slice[Literal['b'], type[str], None]` [not-a-type]
- ERROR python/pyspark/pandas/tests/test_typedef.py:208:37-47: Expected a type form, got instance of `slice[Literal['a'], type[float], Literal['a'] | type[float]]` [not-a-type]
+ ERROR python/pyspark/pandas/tests/test_typedef.py:208:37-47: Expected a type form, got instance of `slice[Literal['a'], type[float], None]` [not-a-type]
- ERROR python/pyspark/pandas/tests/test_typedef.py:208:49-57: Expected a type form, got instance of `slice[Literal['b'], type[int], Literal['b'] | type[int]]` [not-a-type]
+ ERROR python/pyspark/pandas/tests/test_typedef.py:208:49-57: Expected a type form, got instance of `slice[Literal['b'], type[int], None]` [not-a-type]