fix: manual config not working for ChartJS charts

[HardeepAsrani]

Summary

Move override() to after settings.plugins.tooltip is assigned and before new Chart(), so manual config is applied last. Also change $.extend to deep merge so nested plugin options merge rather than replace, preserving tooltip callbacks.

Will affect visual aspect of the product

YES/NO

No

Screenshots

Test instructions

• Make sure the Chart configs work.

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 #1184.

[pirate-bot]

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