Route valid_if through memory resources

[nirandaperera]

Description

A part of #20780.

• Port cudf::detail::valid_if to take cudf::memory_resources and route scan scratch separately from the returned null mask.
• Update validity-mask construction callers to pass output and temporary resources explicitly.
• Add allocation-routing coverage using BaseFixtureWithHarness.

Test plan

• Pre-commit checks on all changed files
• Build BITMASK_TEST, LISTS_TEST, and GROUPBY_TEST
• BITMASK_TEST (54/54 passed)

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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