Generate test gRPC transport address per application context

[filmi7873]

This updates TestGrpcTransportAutoConfiguration to generate the in-process test gRPC transport address as an application-context bean rather than a static field.

Previously, the generated address was shared for the lifetime of the JVM. When multiple application contexts were created in the same test run, such as with nested tests using different profiles, the second context could attempt to register an in-process gRPC server with the same name and fail with java.io.IOException: name already registered.

The generated address is now context-specific and injected into both the test server factory and test channel factory, so each application context gets a unique address while the server and client within the same context still share one.

A regression test has been added to start two contexts while the first remains active.

Fixes #50860

Tests:

• ./gradlew :module:spring-boot-grpc-test:format
• ./gradlew :module:spring-boot-grpc-test:test --tests "*TestGrpcTransportAutoConfigurationTests"
• ./gradlew :module:spring-boot-grpc-test:test --tests "*Grpc*"

[snicoll]

Thanks for the PR but I don't think this is the right way to address this. This would create a new server irrespective of whether it is needed. I think the auto-configuration and its slice does not integrate to the context as they should.