[adenzhou1350]

Reading a block beyond EOF can turn the requested length into a negative number. For example, fs.read_block(path, 5, 1) on a three-byte local file passes -2 to the file's read() and raises ValueError, whereas the memory backend returns an empty block. The same problem affects length=None and delimiter reads.

Clamp the remaining length to zero. Reads within the file and delimiter alignment remain unchanged. Add local/memory regression cases for in-file, EOF, one-byte-past and two-byte-past offsets, with finite/None lengths and with/without a delimiter.

Validation against master 94afb3f6:

• Unmodified source: 4 failures / 28 passing controls on both Windows and Linux Python 3.12.
• Fixed: Windows utility module 165 passed; Linux utility, filesystem-spec, local and memory modules 781 passed, 28 skipped, 1 existing xfail. One existing multithreaded-fork deprecation warning remains.
• Repository-pinned Ruff 0.14.3 check/format and diff check pass.

This is distinct from #2256's split_before handling and does not alter that utility branch. AI-assisted, independently reproduced with actual local files and the public filesystem API. No cloud-service or full-CI qualification is claimed.