[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/Fixture/request_timeout.php

@@ -0,0 +1,42 @@
+<?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.
+ */
+
+/*
+ * Server for {@see \Mcp\Tests\Integration\RequestTimeoutTest}.
+ *
+ * `slow` outlives the client's request timeout and then goes idle again, which
+ * is what lets the test call `fast` on a connection the client has already
+ * given up on once.
+ */
+
+use Mcp\Server;
+use Mcp\Server\Transport\StdioTransport;
+
+require_once dirname(__DIR__, 3).'/vendor/autoload.php';
+
+Server::builder()
+    ->setServerInfo('integration-server', '1.0.0')
+    ->addTool(
+        static function (): string {
+            sleep(2);
+
+            return 'late';
+        },
+        name: 'slow',
+        description: 'Answers well after the client stopped waiting.',
+    )
+    ->addTool(
+        static fn (): string => 'quick',
+        name: 'fast',
+        description: 'Returns at once.',
+    )
+    ->build()
+    ->run(new StdioTransport());

tests/Integration/Fixture/retry.php

@@ -0,0 +1,46 @@
+<?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.
+ */
+
+/*
+ * Server for {@see \Mcp\Tests\Integration\RetryTest}.
+ *
+ * Dies before speaking a frame while the spawn counter is below the configured
+ * number of failures, so the counter file records how many processes the client
+ * actually started.
+ */
+
+use Mcp\Server;
+use Mcp\Server\Transport\StdioTransport;
+
+require_once dirname(__DIR__, 3).'/vendor/autoload.php';
+
+$counter = getenv('MCP_SPAWN_COUNTER');
+
+if (!is_string($counter) || '' === $counter) {
+    exit(1);
+}
+
+$spawns = 1 + (int) file_get_contents($counter);
+file_put_contents($counter, (string) $spawns);
+
+if ($spawns <= (int) getenv('MCP_FAIL_SPAWNS')) {
+    exit(1);
+}
+
+Server::builder()
+    ->setServerInfo('integration-server', '1.0.0')
+    ->addTool(
+        static fn (): string => 'quick',
+        name: 'fast',
+        description: 'Returns at once.',
+    )
+    ->build()
+    ->run(new StdioTransport());