[docs] decision: browser user prompts are handled through a typed handler API

[AutomatedTester]

💥 What does this PR do?

Proposes a design decision record: browser user prompts are handled through a typed handler API.

JavaScript dialogs (alert/confirm/prompt/beforeunload) block the page until handled. WebDriver Classic only exposes them reactively via switch_to.alert, which races the dialog opening and can't pre-decide behaviour. BiDi models them properly (userPromptOpened/handleUserPrompt/userPromptClosed + the unhandledPromptBehavior capability). This record proposes a consistent handler API across bindings: a persistent and a one-shot handler that receive a typed UserPrompt (.type/.message/.default_value/.accept(text)/.dismiss()), with dismiss-by-default when no handler is registered so a stray dialog never hangs the session. The static capability route stays for fixed session policies; expect_user_prompt stays for the assert-it-appeared case.

🔧 Implementation Notes

• Follows the docs/decisions/ process from [docs] add design decision record process and template #17665 — depends on [docs] add design decision record process and template #17665 landing (adds only docs/decisions/0002-*.md).
• Builds on the expect_* primitive proposed in [docs] decision: BiDi events are awaited with expect_* context managers #17671 (record 0001).
• No wire-protocol change; the prompt events/commands already exist in the spec and browsers.
• The dismiss-by-default rule is a cross-binding behaviour commitment and is flagged as such in the Consequences section.

🤖 AI assistance

• AI assisted (complete below)
  • Tool(s): Claude Code
  • What was generated: Initial draft of the decision record, derived from a CDDL-validated comparison of the Python BiDi surface against Playwright; revised through discussion and review
  • I reviewed all AI output and can explain the change

💡 Additional Considerations

One of a set of BiDi ergonomics decision records proposed together. Cross-binding convergence is tracked in the record's binding-status table.

🔄 Types of changes

• Documentation (design decision record)