[Feat]: Add async context manager support to EventQueue

[cchinchilla-dev]

Is your feature request related to a problem? Please describe.

EventQueue has a sophisticated close() with graceful/immediate modes, child propagation, cross-version handling, and idempotency — but it doesn't support async with.

Server-side code that creates and consumes queues must use explicit try/finally or risk leaking resources if an exception occurs between creation and cleanup:

queue = EventQueue()
try:
    await queue.enqueue_event(event)
    ...
finally:
    await queue.close()

Describe the solution you'd like

Add __aenter__ and __aexit__ as concrete methods on EventQueue:

async def __aenter__(self) -> 'EventQueue':
    return self

async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
    await self.close()

This enables the idiomatic pattern:

async with EventQueue() as queue:
    await queue.enqueue_event(event)
    ...
# close() called automatically, even on exceptions

__aexit__ delegates to close() with default immediate=False (graceful), which is the safe default — it waits for the queue to drain and propagates to children. Code that needs immediate shutdown can still call await queue.close(immediate=True) explicitly.

Describe alternatives you've considered

• Do nothing: try/finally works but is verbose and easy to forget. The pattern is already established on the client side (ClientTransport feat: add async context manager support to ClientTransport #682, BaseClient feat: add async context manager support to BaseClient #688, Client feat(client): expose close() and async context manager support on abstract Client #719).
• Add an immediate parameter to __aexit__: This would require storing state set at construction time. Overcomplicates the design for a rare use case — code that needs immediate=True can call close(immediate=True) directly.

Additional context

This follows the same pattern established across the client hierarchy:

• ClientTransport — feat: add async context manager support to ClientTransport #682
• BaseClient — feat: add async context manager support to BaseClient #688
• Client — feat(client): expose close() and async context manager support on abstract Client #719

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