Social: use connection template in per-network editor

[gmjuhasz]

Fixes SOCIAL-453

Proposed changes

• Use connection-level templates to prefill per-network message fields in the block editor when no per-post connection override is set.
• Add per-network fallback help text that clarifies what will be used when the field is empty:
  • Connection template
  • Global template
  • Default network template
• Gate the new per-network display behavior behind social-message-templates.
• Plumb optional messageHelp/messagePlaceholder support from SharePostForm to MessageBoxControl.
• Use the existing messageTemplate setting from social script data/store fallback instead of duplicating the PHP default template in JS.
• Add tests for per-network prefill/help behavior and for explicit MessageBoxControl help override.
• Add a Publicize changelog entry.

Related product discussion/links

• Linear: https://linear.app/a8c/issue/SOCIAL-453/use-connection-template-in-the-editor

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

No.

Testing instructions

• Ensure site features include social-enhanced-publishing and social-message-templates.
• Open Gutenberg editor for a post and open the Social customize-and-preview UI.
• Enable per-network customization.

1. Connection-template prefill

• Set a connection template for one connection in connection management.
• Open that connection in per-network customization.
• Verify the Message field is prefilled with the connection template.
• Verify help text reads: Connection template will be used if empty.

2. Global-template fallback helper

• Use a connection with no connection template.
• Set a non-empty global message template in Social settings.
• Verify helper reads: Global template will be used if empty.

3. Default-network fallback helper

• Save an empty global message template in Social settings.
• Use a connection with no connection template.
• Verify helper reads: The default network template will be used if empty.

4. Override precedence

• Enter a non-empty per-network message override.
• Verify the entered value is shown instead of the connection template.

5. Feature-flag-off regression

• Disable social-message-templates.
• Verify current behavior is preserved (no new fallback helper text and no template-prefill behavior).

6. Automated checks run

• pnpm --filter @automattic/jetpack-publicize test -- per-network.test.tsx message-box-control/tests/index.test.js
• pnpm --filter @automattic/jetpack-publicize typecheck
• pnpm --filter @automattic/jetpack-publicize run build-production-concurrently

[github-actions]

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[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/publicize/_inc/components/customize-and-preview/customization-forms/per-network.tsx	30/42 (71.43%)	71.43%	-14 💚

Full summary · PHP report · JS report

[manzoorwanijk]

I think we should avoid re-defining this and we should rather use messageTemplate from the script data that I added in #48560, as it reads the saved settings template.

[gmjuhasz]

Updated in 68910cc. I removed the duplicated DEFAULT_MESSAGE_TEMPLATE JS constant and now treat getSocialSettings().messageTemplate as the global template layer. That selector already falls back to the social script-data settings.messageTemplate when REST settings are not loaded, so an empty saved global template now falls through to the network default without hardcoding the PHP default in JS.

[manzoorwanijk]

Works great

[manzoorwanijk]

Merging this to rebase #48573