Align remote file reads with IO length semantics

[OskarEichler]

Problem: Net::SFTP::Operations::File#read did not follow Ruby IO length semantics: it accepted invalid negative lengths, performed a remote read for zero, returned an empty string instead of nil at positive-length EOF, and could spin when a server returned an empty DATA packet.

Fix: Coerce lengths through to_int, reject negative lengths, return immediately for zero, return nil at EOF for positive reads, and treat empty DATA as EOF.

Regression coverage: Adds focused upstream cases for coercion, invalid lengths, zero-length reads, EOF, and empty DATA. test_file.rb: 38 runs / 120 assertions. Full suite: 439 runs / 1,210 assertions with only the pre-existing Ruby 4 StartTest positional-hash/keyword expectation failure.

[OskarEichler]

I did another pass over the read state machine and added a regression ensuring that once EOF has been observed, subsequent positive-length reads return nil without issuing another SFTP READ. The focused file tests pass (38 runs, 121 assertions). The full suite otherwise reaches 438/439 passing locally; the sole failure is the pre-existing StartTest#test_with_block_and_options keyword/hash expectation under this Ruby/net-ssh combination.