Forms: add "is exporting" state to the export endpoint handler

[simison]

Implements loading state suggestion from #45275 (comment)

There's a weird delay before the window shows up after button returns to normal, and button doesn't quite look like isBusy looks at docs:

Screen.Recording.2025-09-25.at.12.47.08.mov

Docs button:

Screen.Recording.2025-09-25.at.12.49.33.mov

Proposed changes:

• Adds "isExporting" alongside onExport and use it in the download button

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Testing instructions:

• Go to '..'

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bin/jetpack-downloader test jetpack forms/add-rest-export-endpoint-isExporting

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Code Coverage Summary

Coverage changed in 4 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/forms/src/contact-form/class-contact-form-endpoint.php	620/755 (82.12%)	-6.68%	62 💔
projects/packages/forms/src/contact-form/class-contact-form-plugin.php	425/1318 (32.25%)	-0.24%	12 💔
projects/packages/forms/src/dashboard/hooks/use-export-responses.ts	0/45 (0.00%)	0.00%	3 ❤️‍🩹
projects/packages/forms/src/dashboard/inbox/export-responses/csv.tsx	0/7 (0.00%)	0.00%	-9 💚

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}

[enejb]

I am closing this PR since I cherry-picked these changes to the original PR. Thanks @simison !