NotStartsWith residual evaluation always returns false

[kevinjqliu]

ResidualVisitor.visit_not_starts_with appears to always return AlwaysFalse() for partitioned residuals because it applies Python not to the AlwaysTrue() / AlwaysFalse() object returned by visit_starts_with.

Example with an identity partition:

partition value: x_part = "bb"
filter: NotStartsWith("x", "a")
actual residual: AlwaysFalse()
expected residual: AlwaysTrue()

This can attach an incorrect residual to planned scan tasks.

Relevant code: pyiceberg/expressions/visitors.py ResidualVisitor.visit_not_starts_with.

[kevinjqliu]

Verified against the current worktree.

StartsWith("x", "a") on identity partition "aa" -> AlwaysTrue()
NotStartsWith("x", "a") on identity partition "aa" -> AlwaysFalse()
NotStartsWith("x", "a") on identity partition "bb" -> AlwaysFalse()  # expected AlwaysTrue()

Root cause is ResidualVisitor.visit_not_starts_with: it applies Python not to an AlwaysTrue() / AlwaysFalse() expression object, and both objects are truthy.