docs: describe per-test Vaadin configuration for browserless tests

[mshabarov]

Summary

Browserless Test 1.2 lets a test set Vaadin application properties, feature flags, and Lookup services for one test class or one test method, with the new @BrowserlessTestConfig annotation or the matching builder methods (vaadin/browserless-test#148). Nothing in the documentation mentions it, and the Configuring the Extension table lists only withServices(...).

The feature replaces a pattern the docs never endorsed but that tests had to use: setting vaadin.experimental.* system properties in @BeforeAll and clearing them in @AfterAll. Flags set through the annotation need no development mode and write nothing into the project folder.

What changed

• New page articles/flow/testing/browserless/test-configuration.adoc (order 46, right after JUnit 6 Extensions):
  • the annotation on a class and on a method, with the settings table for applicationProperties, featureFlags, and lookupServices;
  • the merge rules — every inherited annotation contributes, the closest declaration wins, lookup services accumulate, and a method-level annotation is rejected when BrowserlessClassExtension shares one environment across the class;
  • the programmatic surfaces: the extensions, the application context builder used by multi-user tests, and a testConfiguration() override, with the precedence between them;
  • a note that a Vaadin property from the Spring environment is applied on top by SpringServlet and therefore wins.
• extensions.adoc — three rows for the new builder methods, and a line pointing at the new page.

Notes for the reviewer

• vaadin/browserless-test#148 is merged but landed after the 1.2.0-rc1 tag, so it needs 1.2.0 final to ship with 25.3. Hold this pull request if the release does not pick it up. The API carries @since 25.3 in the sources.
• The same pull request moved the services that the Spring and Quarkus integrations need from lookupServices() to frameworkLookupServices(), and deprecated lookupServices(). That is a behavior change for a test that overrides it, and belongs in the upgrade guide rather than here.
• The page is new rather than a section of extensions.adoc, because the annotation works for plain BrowserlessTest subclasses too, and those tests never read the extensions page.

How to test

Documentation only, with inline snippets that are not compiled. The behavior matches BrowserlessTestConfig, BrowserlessConfiguration, and the configuration tests on vaadin/browserless-test main.

🤖 Generated with Claude Code

[mcollovati]

A configuration built on an extension or on the application context builder wins over the class-level annotation, and loses against the method-level one

This is true only for the JUnit extension. BrowserlessApplicationContext only uses the configuration built with its own builder; it does take into account any annotation.
However, the annotation configuration can be provided explicitly by using BrowserlessConfiguration.from(...) factory method.

BrowserlessApplicationContext.create(b -> b.withViewPackages(MyView.class)
        .withConfiguration(BrowserlessConfiguration.from(getClass())));