Handle Struct members that override the indexing method

[OskarEichler]

Summary

Use the core Struct index reader for members. A valid Struct member named [] overrides the reader and currently raises ArgumentError while printing.

Reproduction and verification

PP.singleline_pp(Struct.new(:[], :other).new(1, 2), String.new) raises before this patch. The corpus also covers send, class, members, size and to_a member names.

• Isolated branch: 6 focused checks pass under rbenv Ruby 4.0.6.
• Existing upstream RBENV_VERSION=4.0.6 rbenv exec rake test: 36 tests / 56 assertions, zero failures before and after.
• 2,000 ordinary single-/multi-line outputs remain byte-identical.
• Source-only change; no test files added or modified under the contributor's task constraint. Focused scripts are outside the repository. Other Ruby engines/versions and native Windows were not executed.

Breaking-change notes

No API removal. Struct member access now uses core storage even when [] is overridden, consistent with the existing core lookup for members and class. A member named [] now prints successfully.