fix: fix regex API prefix substitution for dataset selection

[atarashansky]

• Fixes an issue with the new REGEX not properly updating the API prefix when selecting a new dataset
• Fixes an issue with trailing slashes when trying to access a CXG on localhost:3000 (this broke local e2e tests).

The "REGEX_PATHNAME_FOR_DATASET_SELECTOR" is what the original regex was prior to the changes made for enabling access to CXGs in S3 subfolders

Tested in a pirated dev deployment since we do not have test infrastructure for testing dataset selection.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.74%. Comparing base (b7d377b) to head (49779fa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #807   +/-   ##
=======================================
  Coverage   77.74%   77.74%           
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  Files          88       88           
  Lines        6802     6802           
=======================================
  Hits         5288     5288           
  Misses       1514     1514           

Flag	Coverage Δ
frontend	77.74% <ø> (ø)
javascript	77.74% <ø> (ø)
smokeTest	∅ <ø> (∅)
unitTest	77.74% <ø> (ø)

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[tihuan]

LGTM after adding the comments to capture context for the code 🥇 Thanks so much, Alec 🙏 !!