Avoid unneeded shutdown timer on already closed connection

[clue]

This changeset updates the SOCKS server to avoid calling endConnection() when the connection is no longer writable. This avoids creating a useless 3s shutdown timer when the SOCKS handshake fails on a connection that is already closing.

Builds on top of #121

[Copilot]

Pull request overview

This PR prevents the SOCKS server from invoking endConnection() when the incoming connection is no longer writable, avoiding an unnecessary 3s forced-shutdown timer in failure/cancellation scenarios (notably when the client is already closing during handshake).

Changes:

• Guard endConnection() in Server::onConnection() with ConnectionInterface::isWritable().
• Update unit tests to stub isWritable() where the rejection path is exercised.
• Add a regression test covering the “already closed / not writable” scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/Server.php	Adds an isWritable() guard before calling endConnection() on SOCKS handshake failure.
tests/ServerTest.php	Extends mocks with isWritable() and adds a regression test for the closed-connection failure path.

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