[mcollovati]

This PR cherry-picks changes from the original PR #280 to branch 25.3.

Original PR description

Review risk: two-way door — changes a default in the mock environment only; tests that relied on Copilot initializing (unlikely) now need copilot.enable=true.

Summary

Every mock Vaadin environment initializes a real servlet, and when Copilot is on the test classpath its service init listener scans the classpath on each initialization, i.e. once per test. There is no browser and no Copilot UI in a browserless test, so that only slows the suite down. Copilot is now disabled by default in the mock environment, and a test that needs it can enable it again through the regular test configuration.

Refs #278 (Spring is not covered yet, see below)

What changed

• MockVaadin.setupServlet sets copilot.enable=false on the mock context and servlet init parameters before the test's BrowserlessConfiguration application properties. It is a default, not an enforced value: @BrowserlessTestConfig(applicationProperties = "copilot.enable=true") or withApplicationProperty("copilot.enable", "true") re-enables Copilot.
• The key is copilot.enable without a vaadin. prefix, because that is what Copilot reads from ApplicationConfiguration outside Spring. Flow adds the vaadin. prefix only when it looks up JVM system properties.
• Behavior change: Copilot no longer initializes in plain JUnit, extension, BrowserlessApplicationContext and Quarkus based tests unless enabled.
• Not covered: with Spring, Copilot reads vaadin.copilot.enable from the Spring Environment and ignores init parameters, so SpringBrowserlessTest is unaffected. The README explains how to set it there. Options for a framework-level default are discussed on the issue.
• README documents the default and how to override it.

Test summary

• Without configuration, the application configuration reports Copilot disabled (fails without the change).
• A method-level @BrowserlessTestConfig with copilot.enable=true turns it back on, proving the test configuration wins over the default.
• Full build passes (junit6: 1503 tests).

Fixes #278