fix access to core_version in get_flow_run_command()

[jameslamb]

Summary

In #3113 , agents were changed to choose a different flow run command based on the current version of Prefect Core managing the flow. That PR added a default for core_version for the cases where {"core_version": null} exists on the response. However, if core_version is not present on the response at all, you get this hard-to-understand error:

item = 'core_version'

    def __getattr__(self, item):
        try:
            try:
                value = self.__getitem__(item, _ignore_default=True)
            except KeyError:
                value = object.__getattribute__(self, item)
        except AttributeError as err:
            if item == '__getstate__':
                raise BoxKeyError(item) from None
            if item == '_box_config':
                raise BoxError('_box_config key must exist') from None
            if self._box_config['conversion_box']:
                safe_key = self._safe_attr(item)
                if safe_key in self._box_config['__safe_keys']:
                    return self.__getitem__(self._box_config['__safe_keys'][safe_key])
            if self._box_config['default_box']:
                return self.__get_default(item)
>           raise BoxKeyError(str(err)) from None
E           box.exceptions.BoxKeyError: "'GraphQLResult' object has no attribute 'core_version'"

This PR fixes that.

Thanks for your time and consideration!

Changes

Changes to utilities.agent.get_flow_run_command() to use getattr() when looking for core_version in a flow_run.

Importance

I discovered this PR while running the unit tests for a custom KubernetesAgent that I've written with some my-company-specific logic. I was mocking the flow_run contents and hadn't included core_version. Others doing that might face this issue, and this PR will mitigate it.

I'm not that familiar with GraphQL, so I don't know if you're guaranteed to get {"core_version": null} in the response if you ask for that field. If you aren't, then other people running versions of core that don't return this field would also face this issue without this fix.

Checklist

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[codecov]

Codecov Report

Merging #3177 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

[joshmeek]

Thanks @jameslamb!