test(s3): fix flaky EOF check and remove PHP 8.5 skip

[joshtrichards]

• Resolves: #

Summary

Fix S3Test::testFileSizes() so it no longer relies on an invalid EOF behavior assumption and remove the temporary PHP 8.5 CI skip.

Changes

• replace the immediate feof() check after fseek() with a read-at-EOF assertion
• assert seek success explicitly
• use stricter assertSame() checks for byte comparisons
• simplify the chunk-writing loop used to generate test input
• remove the temporary PHP 8.5 skip

Why

readObject() returns a seekable HTTP-backed stream, and feof() is not a reliable thing to assert immediately after seeking to the end of that stream.

Depending on buffering and wrapper behavior, EOF may only be established after a read attempt. Verifying that a read at EOF returns no bytes better matches the behavior this test intends to validate and avoids CI flakiness. The original exclusion by PHP version was for PHP 8.3 (#47388), but recently was changed to 8.5 (#61158). Presumably it's no longer needed (if evidence suggests otherwise going forward we should figure out why / fix the test).

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI