Support string membership in selectattr and rejectattr

[alloutflo]

Summary

• allow CharSequence operands in the in expression test, matching the membership operator's existing behavior and the documented string example
• add the issue's exact rejectattr expression as a regression test
• preserve the existing invalid-argument behavior for non-string, non-iterable operands

Fixes #1227.

Testing

• focused membership, rejectattr, and selectattr tests: 17 passed
• full mvn verify under JDK 23 with Java release 17 and an explicit en_US test locale: 2,217 tests passed, 23 skipped
• Spotless, Checkstyle, SpotBugs, and PMD passed

AI assistance disclosure

OpenAI Codex was used to analyze, implement, and test this change. The diff and checks listed above were verified in the contribution workspace.