docs: add the browserless behavior changes to the upgrade guide

[mshabarov]

Summary

Four changes in Browserless Test 1.2, which is what Vaadin 25.3 ships, break tests that pass today. The TestBench and Browserless Testing section of the upgrade guide mentions none of them, so a suite that starts failing after the upgrade has nothing to point to.

• vaadin/browserless-test#211 — NumberFieldTester, DatePickerTester, TimePickerTester and DateTimePickerTester commit a value that violates min, max, step or a required constraint, instead of throwing. Every assertThrows(IllegalArgumentException.class, ...) around those calls fails.
• vaadin/browserless-test#139 — isUsable() returns false for a read-only value component, so setting a value on a read-only field through its tester now throws.
• vaadin/browserless-test#203 — a component set as a Grid column header, footer or editor is found by a query, where the lookup used to fail.
• vaadin/browserless-test#190 — CheckboxGroupTester.updateSelection is private and no longer appears on the generated locator.

What changed

Four subsections in articles/upgrading/index.adoc, under the existing TestBench and Browserless Testing heading, each saying what changed and what to write instead. The first carries a before-and-after snippet, and links to Component Testers for the full rule.

Notes for the reviewer

• The isUsable() entry documents a change that #6034 already described on the Component Testers page. It is repeated here because it breaks existing tests silently, which is what this guide is for. Drop the subsection if the duplication is unwanted.
• The cross-reference to component-testers#constraint-enforcement points at the section rewritten in docs: cover the browserless testers and methods added in 25.3 #6089, which explains which constraints still make a tester throw. Merging that first keeps the link meaningful, though the anchor exists either way.
• The lookupServices() change from vaadin/browserless-test#148 is not here: that API is still unreleased, and its upgrade note travels with docs: describe per-test Vaadin configuration for browserless tests #6094.

How to test

Documentation only, with an inline snippet that is not compiled. The behavior matches the testers on vaadin/browserless-test main.

🤖 Generated with Claude Code

[mcollovati]

Some more changes that should be mentioned in the upgrade guide:

• feat!: refuse text values the browser physically prevents in TextFieldTester and TextAreaTester browserless-test#252 complements feat!: commit values the browser would commit in value testers browserless-test#211, by throwing exceptions for inputs that the browser does not allow at all.
• feat!: return the component the grid rendered from GridTester.getCellComponent browserless-test#250 prevents GridTester creating a new component instance every timegetCellComponent is called. Old behavior can be kept using renderCellComponent method.

Maybe also worth mentioning that test methods added to the TesterWrapper interface for TreeGridTester and GridContextMenuTester can break compilation, since previously a different overload with a different return type was taken.