fix(admin): decouple AJAX calls and remove unnecessary return values

[faisalahammad]

What?

Closes #131

Decouple the AJAX call chain in the JS admin so each step takes its own arguments instead of relying on raw response data from the previous step. Remove the now-unnecessary return values from the PHP endpoint.

Why?

The previous JS chain passed the full response data object from one step to the next, forcing PHP endpoints to echo back values they did not own. For example, set_up_environment returned plugin and checks only because the next step needed them. This made the flow harder to understand and maintain.

How?

• Added a fetchAJAX(formData) wrapper to reduce repeated fetch boilerplate across 5 call sites.
• Added a getSelectedValues(list) helper to read checked checkboxes from a NodeList.
• Added a module-scoped currentChecks variable so check slugs are stored between getChecksToRun and runChecks.
• Refactored getChecksToRun, setUpEnvironment, runChecks, and runCheck to accept individual parameters instead of a data object.
• Removed plugin and checks from the set_up_environment response. configure_runner() now returns void.
• Updated exportResults to use the shared fetchAJAX wrapper.

Testing Instructions

1. Open Tools > Plugin Check, pick a plugin, click Check it!.
2. Verify results appear as expected. No console errors.
3. Open DevTools > Network, confirm all four AJAX calls return 200 and success: true.
4. Check the plugin_check_set_up_environment response - it should contain only data.message (no data.plugin or data.checks).
5. Click export buttons (CSV, JSON, CTRF, Markdown) - all should download files with the correct filenames.
6. Test failure paths: no plugin selected, invalid nonce via console, subscriber role.

AI Usage Disclosure

• This PR was created without the help of AI tools
• This PR includes AI-assisted code or content

If AI tools were used, please describe how they were used:
Used AI-assisted development tools for code suggestions and refactoring patterns.

Screenshots or screencast

No UI changes. The plugin behaves identically before and after.

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Co-authored-by: faisalahammad <faisalahammad@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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