Add forms mode-selection and TestBench reliability guidance

[peholmst]

What

Adds best-practice guidance to three existing pages:

• building-apps/forms-data/add-form/fields-and-binding.adoc — a "Choosing a Mode" scenario table mapping common form situations (Save/Cancel buttons, multi-step wizard, inline Grid editing, settings panel, filter bar) to buffered vs. write-through Binder mode.
• flow/binding-data/components-binder-beans.adoc — a TIP recommending explicit forField().bind() with method references over bindInstanceFields() for non-trivial forms, and noting that binding by string property name is best reserved for record FDOs.
• flow/testing/end-to-end/reliable-tests.adoc — two new sections: "Keeping Tests Focused and Independent" (reserve end-to-end tests for critical journeys, push coverage down to browserless tests, keep tests independent) and "Waiting for Asynchronous Updates" (wait via waitForFirst()/waitUntil() rather than Thread.sleep()).

Why

This guidance previously lived only in the Vaadin agent skills (forms-and-validation, testbench-testing). Those skills were otherwise restatements of existing docs, so they're being retired; this content — decision tables and best-practice recommendations that a human reader wants too — belongs in the documentation. The docs already covered the underlying mechanics (Binder modes, validators/converters, page objects, the browserless-vs-e2e comparison); these additions fill the gaps in decision and do/don't guidance.

Reviewer notes

• Additive only (46 lines across 3 files); no existing content changed.
• Follows repo conventions: [methodname]/[classname]/[guibutton] macros, cols-based tables, and {articles} xref syntax (verified against existing usage in the repo).
• Coupled change: the agent-skills PR that retires the two skills is Prune skills that only restate the documentation agent-skills#9 — these should land together, since the vaadin-form-layout skill now points readers at these doc guides.
• An unrelated package-lock.json regeneration in the working tree was intentionally left out of this content PR.

🤖 Generated with Claude Code