Handle buffer end offsets as no-op.

[samuel-williams-shopify]

Summary

• add regression coverage for io_read/io_write when offset equals buffer size
• return 0 before touching the IO for zero-capacity buffer operations
• apply the boundary handling consistently to Select, KQueue, EPoll, and URing read/write paths

Verification

• bundle exec rubocop
• bundle exec bake test
• git diff --check origin/main..HEAD

[Copilot]

Pull request overview

This PR tightens boundary handling for buffered IO operations so that an offset equal to the buffer size is treated as a zero-capacity no-op (returning 0) rather than attempting to touch the underlying IO. This aligns behavior across the pure-Ruby Select selector and the native kqueue, epoll, and io_uring implementations, and adds regression coverage to prevent closed-IO side effects.

Changes:

• Add regression tests for io_read/io_write returning 0 when offset == buffer.size.
• Update Select selector Ruby implementation to return 0 for end-offset operations before invoking IO.
• Update kqueue, epoll, and io_uring native paths to treat offset == size as a no-op and avoid descriptor/IO access in that case.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
test/io/event/selector/buffered_io.rb	Adds regression tests covering offset == buffer.size returning 0 for read/write.
lib/io/event/selector/select.rb	Adds explicit end-offset handling (offset == buffer.size => 0) for Select-based read/write.
ext/io/event/selector/uring.c	Ensures end-offset returns 0 before descriptor/seek handling in io_uring read/write paths.
ext/io/event/selector/kqueue.c	Refactors kqueue buffered read/write to early-return 0 on end-offset and operate on the remaining region.
ext/io/event/selector/epoll.c	Refactors epoll buffered read/write similarly to handle end-offset as a no-op consistently.