fix: visualizer getCSV method throwing constant errors

[HardeepAsrani]

Summary

Adds safeguards to prevent the _getCSV from throwing constant errors.

Will affect visual aspect of the product

YES/NO

Screenshots

Test instructions

• Not sure how we can test, we have just added safe guards to prevent it from happening.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/visualizer-pro/issues/400.

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[Copilot]

Pull request overview

This PR aims to stop Visualizer’s CSV export (_getCSV) from repeatedly triggering runtime errors by adding additional safeguards around temporary file creation, and updates the test suite/tooling to support newer PHPUnit behavior.

Changes:

• Add a fallback/early-return guard in Visualizer_Module::_getCSV() when a writable temp file handle can’t be created.
• Refactor/export tests to reuse a helper for running the export AJAX action and add CSV response-structure assertions.
• Add Yoast PHPUnit Polyfills (and bootstrap loading) to improve PHPUnit cross-version compatibility; update composer.lock accordingly.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
classes/Visualizer/Module.php	Adds additional guard(s) when temp file creation fails during CSV generation.
tests/test-export.php	Adds a helper to run export AJAX, adds CSV structure test, adjusts invalid-chart expectations.
tests/test-ajax.php	Replaces deprecated PHPUnit assertions with property_exists() checks.
tests/bootstrap.php	Loads Yoast PHPUnit Polyfills autoloader early in the test bootstrap.
composer.json	Adds yoast/phpunit-polyfills to require-dev.
composer.lock	Locks new dev dependencies brought in by PHPUnit polyfills/tooling changes.

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[Soare-Robert-Daniel]

I think Copilot made some great points on the review.

[Copilot]

Pull request overview

Copilot reviewed 5 out of 6 changed files in this pull request and generated 2 comments.

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