[py] Add expect_* context managers for BiDi events

[AutomatedTester]

Motivation and Context

Waiting for a BiDi event today requires registering a callback, performing the action, polling with WebDriverWait, and removing the callback by hand — and if the event fires between the action and the first poll registration, it can be missed entirely. This PR adds Playwright-style expect_* context managers that register the event handler before the triggering action runs, eliminating that race and the boilerplate:

with driver.network.expect_response("**/api/**") as response_info:
    driver.find_element(By.ID, "load").click()
response = response_info.value
assert response.status == 200

What's added

Core primitive (py/private/_event_manager.py):

• Subscription — queue-based capture of the first matching event, .wait(timeout) / .value, cancel(), context-manager protocol that waits for the event on clean with-block exit (raising TimeoutException if it never arrives), idempotent lock-guarded detach.
• _EventManager.expect(...) plus a raw=True option on add_event_handler so handlers can receive the full wire-level params dict where the generated event dataclasses drop fields (e.g. request/response). Not exposed on the generated modules' public API.

High-level methods (generated via the enhancements manifest):

• driver.network.expect_request(url_or_predicate) / expect_response(url_or_predicate) — URL glob (*, **, ?) or predicate filtering, wrapping the existing Request/Response classes (observational; no intercept is created).
• driver.script.expect_console_message(predicate) — built on the existing LogHandlerRegistry, captures ConsoleMessage.
• driver.browsing_context.expect_user_prompt(predicate) — captures typed UserPromptOpenedParameters; the result's .context feeds straight into handle_user_prompt().
• driver.browsing_context.expect_download() — correlates downloadWillBegin/downloadEnd by navigation id into a new Download object with suggested_filename, path(), save_as() and failure().

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

No existing public signatures changed; all additions are new methods.

Testing

• 25 new unit tests (py/test/unit/.../bidi_expect_events_tests.py) covering capture, glob/predicate filtering, timeout (including timeout=0), predicate exceptions, cached value, idempotent/concurrent cancel, detach-after-capture, cross-thread wait, and download correlation/cleanup.
• 6 new browser-based tests appended to bidi_network_tests.py, bidi_script_tests.py and bidi_browsing_context_tests.py — all passing on Chrome locally (expect_download is xfail_firefox, matching the existing download tests).
• Existing bidi_network / bidi_script unit suites pass; ./scripts/format.sh clean.