[Server] Record the missing symfony/finder failure as a BC break

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: ousamabenyounes

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 All notable changes to `mcp/sdk` will be documented in this file.
 
+0.10.0
+------
+
+* [BC Break] `Builder::build()` throws a `RuntimeException` when `setDiscovery()` is used without `symfony/finder` and without a custom discoverer, instead of logging a warning and discovering nothing
+
 0.9.0
 -----
 

tests/Unit/Server/BuilderTest.php

@@ -97,6 +97,7 @@ public function testBuildWithoutCustomReferenceHandler(): void
     #[TestDox('build() fails when file-based discovery is configured without Symfony Finder')]
     public function testBuildFailsWhenDiscoveryDependencyIsMissing(): void
     {
+        // Load every class the failing path needs before the Composer autoloaders are unregistered.
         Server::builder()->setServerInfo(self::TEST_SERVER_NAME, self::TEST_SERVER_VERSION)->build();
         Server::builder()
             ->setServerInfo(self::TEST_SERVER_NAME, self::TEST_SERVER_VERSION)