E2E test: SASL mechanism validation — unsupported mechanism rejection

[s2x]

Closes #148

Problem

No E2E test verifies that the server correctly rejects an unsupported SASL mechanism. The ConnectionHandshakeTest only tests PLAIN authentication. If a client sends a mechanism name the server doesn't support (e.g. "SCRAM-SHA-256", "EXTERNAL", or an empty string ""), the server should respond with SASL_MECHANISM_NOT_SUPPORTED (0x07).

Expected test

public function testUnsupportedSaslMechanismThrows(): void
{
    $connection = $this->createConnection();
    $connection->sendMessage(new PeerPropertiesToStreamBufferV1());
    $connection->readMessage();
    $connection->sendMessage(new SaslHandshakeRequestV1());
    $connection->readMessage();

    $this->expectException(\Exception::class);
    $connection->sendMessage(new SaslAuthenticateRequestV1('SCRAM-SHA-256', 'guest', 'guest'));
    $connection->readMessage(timeout: 2.0);
}

Why it matters

• Validates that the client library correctly propagates SASL errors
• Ensures ResponseCodeEnum::SASL_MECHANISM_NOT_SUPPORTED path is exercised
• Security: confirms the server enforces mechanism restrictions

Acceptance criteria

• Test with a non-existent SASL mechanism name → exception thrown
• Test with empty string mechanism → exception thrown
• Verify the exception message contains meaningful error info