test(ext-mcp): migrate McpAuthPolicyTest to flash-testing

First migration, chosen because it is the case that drove the harness design:
a Flash app plus a FakeOidcProvider, with tokens audience-bound to the app's
own port, so the port has to be readable after boot.

Drops the racy free-port dance, the hand-rolled HttpClient plumbing and the
per-test teardown; failures now report the response body. 120 to 106 code
lines, and what remains is tool-policy assertions rather than fixture code.

The two boot-rejection tests keep building their app directly — a harness whose
job is to boot an app is the wrong tool for asserting that booting fails — but
port(0) removes freePort() from those too.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

flash-extensions/flash-ext-mcp/pom.xml

@@ -38,6 +38,11 @@
             <groupId>org.junit.jupiter</groupId>
             <artifactId>junit-jupiter</artifactId>
         </dependency>
+        <dependency>
+            <groupId>dev.relism</groupId>
+            <artifactId>flash-testing</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>