[Server] Make CollectingLogger test fixture compatible with psr/log v1

The anonymous logger in StreamableHttpTransportTest used the v2/v3 method
signature `log($level, \Stringable|string $message, ...)`. Project
composer.json allows psr/log ^1.0 || ^2.0 || ^3.0, so on `--prefer-lowest`
CI installs v1 which keeps the untyped parameter and fatals on LSP
incompatibility. Drop the type hint to keep the fixture working across
all three major versions.

Author: sveneld

tests/Unit/Server/Transport/StreamableHttpTransportTest.php

@@ -285,7 +285,8 @@ private function collectingLogger(): object
             /** @var list<array{level: string, message: string}> */
             public array $records = [];
 
-            public function log($level, \Stringable|string $message, array $context = []): void
+            // Signature kept untyped on $message to stay compatible with psr/log v1 LoggerInterface.
+            public function log($level, $message, array $context = []): void
             {
                 $this->records[] = ['level' => (string) $level, 'message' => (string) $message];
             }