jetpack: Fix undefined array warnings in Donations block

[anomiex]

Proposed changes:

If the donations block sets oneTimeDonation, monthlyDonation, and/or annualDonation without including a planId field, ensure the field is set by including the value used in the block.json in the defaults.

If the currency attribute is not set or is invalid, and showCustomAmount is true, fall back to the generic currency symbol ¤ like Jetpack_Currencies::format_price() already does and default to ¤100.

Also, Jetpack_Currencies::format_price() should honor its $symbol argument when doing that fallback.

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?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

p1720547616141829/1720546514.458029-slack-C01U2KGS2PQ
p1HpG7-tpT-p2

Does this pull request change what data or activity we track or use?

Testing instructions:

• Connect Stripe if necessary. Make sure normal donations blocks work.
• Create a post. Enable the code editor, paste in <!-- wp:jetpack/donations {"oneTimeDonation":{"show":true,"amounts":[5,15,100]}} /--> as a block, and save without going back to the visual editor.
• View the post. Check your PHP logs.

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[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WordPress.com Simple site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin, and enable the fix/jetpack-donations-block-missing-array-keys branch.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack fix/jetpack-donations-block-missing-array-keys
  

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[fgiannar]

LGTM! Thanks for working on this and adding a unit test too.

Not a blocker but I'd also consider adding ¤ as a class constant in Jetpack_Currencies.