[docs] decision: BiDi events are awaited with expect_* context managers

[AutomatedTester]

💥 What does this PR do?

Proposes a design decision record: BiDi events are awaited with expect_* context managers.

Today the only way to react to a BiDi event is a fire-and-forget callback. To wait for an event triggered by a user action, users must subscribe, act, then wait — which races (the event can fire before the wait starts) and forces everyone to re-implement capture-and-filter boilerplate. This record proposes a race-free, predicate-filtered expect_* family (arm-before-action context managers) over a small reusable Subscription primitive, consistent across all five bindings, as the foundation the other event-driven decisions (navigation, downloads, user prompts) build on.

🔧 Implementation Notes

• Follows the docs/decisions/ process and template introduced in [docs] add design decision record process and template #17665 — depends on [docs] add design decision record process and template #17665 landing (this PR adds only docs/decisions/0001-*.md; no conflict with the template/README files).
• The decision is language-neutral and normative across bindings; Python is the reference implementation (the expect_* + Subscription work is implemented and a code PR is pending).
• No wire-protocol change: the backing BiDi events already exist and are emitted by current browsers.

🤖 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

Part of a set of BiDi ergonomics decision records proposed together (events, user prompts, storage/cookies, network body+timing, emulation, navigation). This one should be considered first since the others build on the expect_* primitive it defines. Cross-binding convergence is tracked in the record's binding-status table.

🔄 Types of changes

• Documentation (design decision record)