Fix race condition when fetching stops on rapid zoom/pan

[Sundaresan-Karunakaran]

This PR fixes the issue #232 where rapidly zooming/panning caused a race condition, leading to incorrect outputs and inefficiency.

Problem

When users rapidly zoom in and out or pan the map, multiple requests to fetch stops are triggered in quick succession. This can lead to:

• A race condition where older requests resolve after newer ones, causing outdated or inconsistent data to be displayed.
• Inefficiencies due to unnecessary network requests fetching data that is no longer needed.

Solution

• Implemented an AbortController to cancel previous fetch requests before making a new one.
• Moved the cancellation logic to loadStopsAndAddMarkers to ensure every request is controlled properly.
• Ensured only the latest valid response updates the stop data.

[coveralls]

coverage: 16.097%. remained the same
when pulling 50b574d on Sundaresan-Karunakaran:main
into d4b0cd3 on OneBusAway:main.

[Sundaresan-Karunakaran]

@aaronbrethorst
Apologies, I attempted to squash the commits before submitting, but I couldn't find the option here as I did in my forked repo. Next time, I'll handle it from the CLI directly to make it easier.

Sorry for any confusion!

[Copilot]

Copilot wasn't able to review any files in this pull request.

[Sundaresan-Karunakaran]

Hi @aaronbrethorst, I saw that the PR was closed, but I didn’t get any feedback before that. I just want to understand if there’s anything I could improve or if it was closed for another reason. Appreciate your time!