[Client] Implement setMaxRetries connection retries (#413)

* [Client] Implement setMaxRetries connection retries

* [Client] Reject a negative retry count in Configuration

* [Client] Use a positive init timeout in the retry test

* [Client] Drop redundant docblock note on setMaxRetries

* [Client] Clear the initialized flag on a failed connection

* [Client] Trim comments to the non-obvious ones

* [Client] Cover retries and timeout cleanup with integration tests

The retry fixture counts the processes it was started as, which is what
separates a real retry from a second call into the same server. The timeout
fixture outlives the client's patience and then goes idle, so the next
request runs on a connection the client already gave up on once.

tests/Integration/RequestTimeoutTest.php

@@ -0,0 +1,43 @@
+<?php
+
+/*
+ * This file is part of the official PHP MCP SDK.
+ *
+ * A collaboration between Symfony and the PHP Foundation.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Mcp\Tests\Integration;
+
+use Mcp\Exception\RequestException;
+use PHPUnit\Framework\Attributes\TestDox;
+
+/**
+ * What a timed-out request leaves behind on the connection.
+ *
+ * @see Fixture/request_timeout.php for the server under test
+ */
+final class RequestTimeoutTest extends IntegrationTestCase
+{
+    #[TestDox('an abandoned response is not handed to the next request')]
+    public function testTimedOutRequestDoesNotLeakIntoTheNext(): void
+    {
+        $client = $this->connect('request_timeout', $this->clientBuilder()->setRequestTimeout(1));
+
+        try {
+            $client->callTool('slow');
+            $this->fail('The slow tool should have outlived the request timeout.');
+        } catch (RequestException) {
+        }
+
+        // The server is still executing the abandoned call and cannot answer
+        // anything until it returns, so the next request has to wait it out.
+        sleep(2);
+
+        // Without the pending request being cleared this answers 'late': the
+        // response the client gave up on, handed to the request after it.
+        $this->assertSame('quick', $client->callTool('fast')->content[0]->text ?? null);
+    }
+}