Avoid another remote read when gets has reached its limit

[OskarEichler]

Summary

Treat an exactly satisfied limit as sufficient buffered data instead of fetching another SFTP chunk.

Reproduction

For "a" * 8192 + "b", file.gets(8192) makes two remote read requests on the baseline even though the first supplies all requested bytes; fixed makes one. gets(0) previously read remotely; fixed returns an empty string without a request.

Verification

• Baseline and this isolated patch: 433 existing tests, 1,201 assertions, zero failures/errors/skips, Ruby 4.0.6 through rbenv, net-ssh 7.3.3, Minitest 5.27.0 and Mocha 2.1.0.
• 888 external assertions compare positive/zero limits, separators, repeated reads and cursor positions with StringIO. Native local SFTP reads pass for zero, 8,192-byte and smaller limits.
• The combined consumer candidate also passes 10,384 external model assertions, 21 local OpenSSH pipe assertions, and the existing suite with frozen string literals enabled and disabled. These combined checks include separately attributed portions of Handle frozen strings #157/ensure ruby 4 compatibility and address some security concerns #162; they are not claimed as this isolated branch's changes.
• Packaging preserves all 37 release paths and dependency metadata. All 26 runtime files compile on Ruby 4.0.6 and parse with Parser::Ruby25. Comparative Lint checks retain the same 33 baseline findings; no all-green lint claim.

Breaking changes and limitations

No signature change. Zero-length reads at EOF now return an empty string consistently. Removing the extra request can delay EOF discovery until a later read, matching the requested read limit. Negative-limit semantics and long-line scanning are separate work.

No repository tests were added or modified under the contribution's no-new-tests constraint; focused repro/model drivers ran outside the repository. An external verification Gemfile supplies compatible test tools and RDoc without changing upstream tooling. Actual older Ruby/JRuby/Windows compatibility and upstream CI have not been verified. No production or SSH connections, version bumps, release-tool changes or unrelated modernization are included. Git builds are not signed by the upstream gem release key.