Refactor: Use type[Event] instead of str to filter events

[malhotra5]

End-user friendly description of the problem this fixes or functionality that this introduces

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

The get_matching_events function in EventStream was comparing event types using string names. This change makes it use actual class types instead, which provides:

Type safety - type checker ensures only Event subclasses are passed
Refactoring safety - class renames will update all references
Better IDE support with code completion
No risk of typos in string names

The changes we made:

1. Changed the type of event_type parameter from str | None to type[Event] | None in both functions
2. Changed the comparison from event.class.name == event_type to isinstance(event, event_type)
3. Serialize event_type from string to Event for FASTApi endpoint

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:ccec685-nikolaik   --name openhands-app-ccec685   docker.all-hands.dev/all-hands-ai/openhands:ccec685

[csmith49]

If you're so inclined, this change actually allows you to tighten up the return value annotation as well:

T = TypeVar('T', Event)
def get_matching_events(
    ...,
    event_type: type[T] | None = None,
    ...
) -> list[T]:
    ...

Or, using the condensed syntax in Python 3.12 (from this PEP):

def get_matching_events[T: Event](
    ...,
    event_type: type[T] | None = None,
    ...
) -> list[T]:
    ...