Email add-on's mail-sending TaskExecutor may break Vaadin application startup

[Flaurite]

Description

The Email add-on publishes its internal mail-sending pool as a Spring TaskExecutor bean (mailSendTaskExecutor in EmailConfiguration).
In a Vaadin-based Jmix application Vaadin selects a single TaskExecutor for its own asynchronous work, and a general-purpose TaskExecutor bean takes part in that selection. As a result:

• when mailSendTaskExecutor is the only TaskExecutor in the context, Vaadin silently uses the mail-sending pool for its UI async work (unexpected contention and pool sizing);
• when any second application- or add-on-provided TaskExecutor is present, the application fails to start.

Steps to reproduce

1. A Jmix application with FlowUI (Vaadin 25) and the Email add-on.
2. Add any second Spring TaskExecutor bean - e.g. a background executor from another add-on or from the application itself.
3. Start the application.

Actual behavior

Startup fails while starting the web server:

Caused by: java.lang.IllegalStateException: Multiple TaskExecutor beans found: [app_MyBeanTaskExecutor, mailSendTaskExecutor]. Please resolve this conflict by either: (1) Providing a single TaskExecutor bean, or (2) Marking the bean to use with Vaadin by: naming it 'VaadinTaskExecutor' (e.g. @Bean("VaadinTaskExecutor")), or applying the @VaadinTaskExecutor qualifier annotation to the bean definition.
	at com.vaadin.flow.spring.SpringVaadinServletService.getExecutor(SpringVaadinServletService.java:173) ~[vaadin-spring-25.2.6.jar:na]
	at com.vaadin.flow.server.VaadinService.initSignalsEnvironment(VaadinService.java:363) ~[flow-server-25.2.6.jar:25.2.6]
	at com.vaadin.flow.server.VaadinService.init(VaadinService.java:329) ~[flow-server-25.2.6.jar:25.2.6]
	at com.vaadin.flow.spring.SpringVaadinServletService.init(SpringVaadinServletService.java:180) ~[vaadin-spring-25.2.6.jar:na]
	at com.vaadin.flow.spring.SpringServlet.createServletService(SpringServlet.java:119) ~[vaadin-spring-25.2.6.jar:na]
	at com.vaadin.flow.server.VaadinServlet.createServletService(VaadinServlet.java:342) ~[flow-server-25.2.6.jar:25.2.6]
	at com.vaadin.flow.server.VaadinServlet.init(VaadinServlet.java:134) ~[flow-server-25.2.6.jar:25.2.6]
	at org.apache.catalina.core.StandardWrapper.initServlet(StandardWrapper.java:832) ~[tomcat-embed-core-11.0.24.jar:11.0.24]
	... 34 common frames omitted

Even with no second executor, Vaadin quietly runs its async work on the mail-sending pool rather than a dedicated one.

Expected behavior

Adding the Email add-on should not, on its own, make Vaadin adopt the mail-sending pool, and should not cause a startup failure when the application or other add-ons define their own executors.