[REF] Refactor dataset size request into separate function

[alyssadai]

• Closes Refactor request for total number of subjects in matching datasets into separate function #234

Changes proposed in this pull request:

• Request for sizes of a list of matching datasets now in a separate function from primary subject query

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[coveralls]

Pull Request Test Coverage Report for Build 7065779048

• 39 of 39 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 99.771%

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Totals
Change from base Build 7052301596:	0.008%
Covered Lines:	873
Relevant Lines:	875

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💛 - Coveralls

[rmanaem]

Thanks for taking care of this @alyssadai!
I think we can remove the TODO above mock_post_query_to_graph in conftest.py and then 🧑‍🍳