Add is_open convenience to GetConfigurationActivationApiResult

[craigmcchesney]

Split out from #20, which noted this as worth pairing with the open-ended activation fix but left it out of #25 to keep that diff minimal ahead of the release.

Now that end_time is optional (#25), an activation can be open-ended — endTime genuinely absent, meaning "still in effect". Callers need to detect that shape, not just create it. The motivating case is the live bridge from #20: to close an interval you must first identify the open one.

Current state

The only way to ask "is this still open" is to reach into the protobuf directly:

still_open = not activation.HasField("endTime")

That is what doc/cookbook/machine-configuration.md currently shows. It works, but it pushes a protobuf presence check into user code for what is a first-class domain question, and it requires the caller to know that absent-vs-zero is the distinction that matters.

Suggested change

Add a convenience to GetConfigurationActivationApiResult (machine_config_client.py:477 neighborhood, alongside the existing configuration_activation property):

@property
def is_open(self) -> Optional[bool]:
    """True if the activation is open-ended (still in effect), None if there is no record."""

Returning Optional[bool] keeps it honest on the error path, where configuration_activation is already None — an is_open of False would wrongly imply a closed record exists.

Worth considering the same accessor on the query/iterate paths, which yield bare common_pb2.ConfigurationActivation objects rather than result wrappers. A module-level helper (is_open(activation) -> bool) may serve both shapes better than a property on one result class; that tradeoff is the main design question here.

Scope

• Additive and independent — no change to request building or the wire format.
• Unit tests for: open-ended record, bounded record, error/no-record case.
• Update the cookbook's "still open" recipe to use it, replacing the raw HasField check.