Clean up temporary MongoDB collections created by `/nmdcschema/linked_instances` API endpoint

[eecavanna]

During a pairing session between me and @dwinston today (PT), we noticed that the nmdc database in the MongoDB server in the dev environment had several collections whose names had the format:

_runtime.tmp.linked_instances.{short_hex_str}.{long_hex_str}

Those were created by the /nmdcschema/linked_instances API endpoint.

nmdc-runtime/nmdc_runtime/api/endpoints/lib/linked_instances.py

Lines 33 to 35 in f4e23e5

	def temp_linked_instances_collection_name(ids: list[str], types: list[str]) -> str:
	"""A name for a temporary mongo collection to store linked instances in service of an API request."""
	return f"_runtime.tmp.linked_instances.{hash_from_ids_and_types(ids=ids,types=types)}.{ObjectId()}"

This ticket is about:

1. Deleting those collections from the database
2. Ensuring the Runtime automatically cleans up those collections in the database in the future

[dwinston]

one option: add a FastAPI BackgroundTasks dependency to the endpoint and use it to clean up sufficiently old temp collections after returning a response.

[eecavanna]

Note: The ObjectId has a generation_time property, which we may be able to use to decode the timestamp from the last segment of the collection name string.

[eecavanna]

I think the guidance @dwinston gave me during our pairing session today (see comments above) is sufficient for me to resolve this, myself.

I'll reassign it to myself and also remove it from the Runtime Q4 squad board (I had only added it there as an additional reminder to check in with @dwinston about it).