test: point apigee-llm-proxy-url at the renamed test method

The apigee-llm-proxy-url surefire execution in core/pom.xml is meant to
run one ApigeeLlmTest method with APIGEE_PROXY_URL set, so that
ApigeeLlm's fallback to that environment variable is tested. It selects
the method by name. df73784 replaced that method with three,
including build_withoutProxyUrlAndEnvVarSet_readsFromEnvironment for
this case, and did not update the execution, which still names
build_withoutProxyUrl_readsFromEnvironment. Since then it runs no tests:

  [INFO] --- surefire:3.5.2:test (apigee-llm-proxy-url) @ google-adk ---
  [INFO] Tests run: 0, Failures: 0, Errors: 0, Skipped: 0

The new method skips itself unless APIGEE_PROXY_URL is set, and this is
the only execution that sets it, so the build has skipped the method
since the change. The build still passes because surefire's
failIfNoSpecifiedTests check only looks for a matching test class, and
ApigeeLlmTest still matches.

Point the execution at the new method name.

Author: innoprej

core/pom.xml

@@ -305,7 +305,7 @@
               <goal>test</goal>
             </goals>
             <configuration>
-              <test>ApigeeLlmTest#build_withoutProxyUrl_readsFromEnvironment</test>
+              <test>ApigeeLlmTest#build_withoutProxyUrlAndEnvVarSet_readsFromEnvironment</test>
               <environmentVariables>
                 <GOOGLE_API_KEY>api-key</GOOGLE_API_KEY>
                 <!-- runs a second variant of the test method -->