Limit SSH client STDERR line buffer to 64 KiB

[clue]

This changeset limits the SSH client STDERR line buffer to 64 KiB, matching clue/reactphp-ndjson#10. The internal LineSeparatedReader processes the SSH client's STDERR one line at a time, so a line without a newline would previously keep growing the buffer without bounds. Exceeding the limit now terminates the SSH client process, so a pending connection attempt rejects with Buffer size exceeded. OpenSSH limits its own log messages to 1 KiB and connection data never passes through this buffer, so this should not affect normal operation.

Builds on top of #50 and #8
Resolves / closes #34
Refs clue/reactphp-ndjson#10

[Copilot]

🟢 Approval recommended

The focused changes have regression coverage across both connectors, with no unresolved review findings.

No open issues 🎉

What changed in this PR

Limits SSH client STDERR buffering to prevent unbounded memory growth from unterminated lines.

Changes:

• Adds a 64 KiB line-buffer limit.
• Terminates SSH processes on STDERR errors in both connectors.
• Adds boundary and overflow regression tests.

File	Description
tests/​Io/​LineSeparatedReaderTest.php	Tests line-length boundaries and overflow handling.
tests/​IntegrationSshSocksConnectorTest.php	Tests SOCKS connection rejection on overflow.
tests/​IntegrationSshProcessConnectorTest.php	Tests process connection rejection on overflow.
src/​SshSocksConnector.php	Terminates the process on STDERR errors.
src/​SshProcessConnector.php	Terminates the process on STDERR errors.
src/​Io/​LineSeparatedReader.php	Enforces the buffer limit and reports overflow.

_{Review effort: Balanced}