Fix SerializationTest failure with Vaadin 25.1.x

[javier-godoy]

Problem

SerializationTest fails with Vaadin 25.1.0-rc2:

java.lang.NoClassDefFoundError: jakarta/servlet/http/HttpSessionBindingListener

Root cause

The test was relying on jakarta.servlet:jakarta.servlet-api being provided transitively through the testbench dependency chain. That chain changed between Vaadin versions:

Vaadin	vaadin-testbench-core	includes vaadin-testbench-unit
25.0.x	10.0.3	✅ → transitively brings jakarta.servlet-api:6.1.0
25.1.0-rc2	10.1.0-rc1	❌ removed

In vaadin-testbench-core:10.0.3, the chain was: vaadin-testbench-unit → vaadin-testbench-unit-shared → jakarta.servlet:jakarta.servlet-api:6.1.0:test. In 10.1.0-rc1, vaadin-testbench-unit was dropped from vaadin-testbench-core.

Fix

Add an explicit test dependency to the v25 profile in pom.xml. flow-server:25.1.0-rc1 declares jakarta.servlet:jakarta.servlet-api:6.1.0 as provided, so that is the correct version:

<dependency>
    <groupId>jakarta.servlet</groupId>
    <artifactId>jakarta.servlet-api</artifactId>
    <version>6.1.0</version>
    <scope>test</scope>
</dependency>

Verified locally: mvn test -Pv25 passes with Vaadin 25.1.0-rc2 after adding this dependency to the v25 profile.

Reported from ecosystem build issue: mstahv/vaadin-ecosystem-build#108