fix: FsNeo4jCSVLoader fails if nodes have disjoint keys

[Spacerat]

Summary of Changes

I discovered a bug where FsNeo4jCSVLoader fails if two nodes of the same type have the same number of attributes, but with different names. This happened to us for DashboardChart, due to this code:

amundsendatabuilder/databuilder/models/dashboard/dashboard_chart.py

Lines 58 to 77 in d91c0c5

	def _create_node_iterator(self) -> Iterator[GraphNode]:
	node_attributes = {
	'id': self._chart_id
	}
	if self._chart_name:
	node_attributes['name'] = self._chart_name
	if self._chart_type:
	node_attributes['type'] = self._chart_type
	if self._chart_url:
	node_attributes['url'] = self._chart_url
	node = GraphNode(
	key=self._get_chart_node_key(),
	label=DashboardChart.DASHBOARD_CHART_LABEL,
	attributes=node_attributes
	)
	yield node

The root cause of this is that FsNeo4jCSVLoader names CSV files by the number of keys the node has. My new test demonstrates the problem. When the first node is loaded, a CSV is created with a column for job. On attempting to load the second node, the loader fails because it cannot find a column in the CSV named pet.

This fixes the problem by making the file key dependent on the actual set of record keys. I actually tried two implementations:

1. The first just concatenates the keys, sorted
2. The second builds a dictionary of fieldset -> ID, and assigns increasing IDs.

I went with the second to avoid excessively long filenames.

Tests

I added a unit test which catches the bug. You can check out the "Add failing test" commit and run make test to observe it.

Documentation

N/A

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[feng-tao]

will take a look this pr as well.

[feng-tao]

lgtm, thanks @Spacerat fixing it!

[Spacerat]

Thanks @feng-tao !