[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.

docs/client.md

@@ -57,10 +57,32 @@ $client = Client::builder()
     ->setClientInfo('My Application', '1.0.0', 'Description of my client')
     ->setInitTimeout(30)      // Seconds to wait for initialization
     ->setRequestTimeout(120)  // Seconds to wait for request responses
-    ->setMaxRetries(3)        // Retry attempts for failed connections
+    ->setMaxRetries(3)        // Retries for failed connections
     ->build();
 ```
 
+### Connection Retries
+
+`setMaxRetries()` controls how often `connect()` retries a failed connection. It
+counts retries rather than attempts, so the default of `3` means one initial
+attempt plus up to three retries — four in total — before the `ConnectionException`
+of the last attempt is rethrown:
+
+```php
+$client = Client::builder()
+    ->setMaxRetries(0)  // Fail on the first failed attempt
+    ->build();
+```
+
+Between two attempts the transport is closed, so a retry never reuses a
+half-established connection: a `StdioTransport` spawns a fresh server process and
+an `HttpTransport` discards the session ID of the failed attempt. Each retry is
+preceded by a short, linearly growing delay (100ms, 200ms, 300ms, …).
+
+Only the connection handshake is retried. Individual requests such as
+`callTool()` are always sent once — retrying them is unsafe as tool calls are not
+necessarily idempotent.
+
 ### Client Information
 
 Set the client's identity reported to servers during initialization: