Add filter to hide AI Content Planner inline banner

[thijsoo]

Context

Site owners might want a way to hide the AI Content Planner inline banner from the post editor without per-post dismissal.

Summary

This PR can be summarized in the following changelog entry:

• Adds the wpseo_enable_ai_content_planner_inline_banner filter to hide the AI Content Planner inline banner from the post editor.

Relevant technical choices:

• The filter gates the Meta_Fields_Presenter('content_planner') call in the metabox: returning false stops the hidden post-meta inputs from rendering at all.
• The editor JS now treats an absent yoast_wpseo_is_content_planner_banner_dismissed input as "banner disabled" by checking the input element's existence instead of comparing its value to "1".
• The "Get content suggestions" sidebar/metabox button is unaffected — only the inline banner is hidden.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

1. On a clean install with the AI Content Planner feature available, edit a new post in the block editor. The "AI Content Planner" inline banner appears above the first block (requires at least 5 published posts).
2. Add the following snippet to a mu-plugin or your theme's functions.php:

   add_filter( 'wpseo_enable_ai_content_planner_inline_banner', '__return_false' );

3. Reload the post editor. The inline banner is no longer rendered, and the hidden input yoast_wpseo_is_content_planner_banner_dismissed is absent from the page source.
4. Open the Yoast sidebar / metabox. The "Get content suggestions" button is still present and functional.
5. Remove the filter. Reload the editor. The banner is back on new posts.
6. Verify dismissed posts: dismiss the banner on a post (with the filter removed), reload — the banner stays hidden for that post. Add the filter, reload — still hidden. Remove the filter, reload — still hidden (per-post dismissal preserved).

Relevant test scenarios

• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

• The AI Content Planner inline banner JS bootstrap (packages/js/src/ai-content-planner/helpers/fields.js) — the dismissal-state seed now reads the input element directly. Per-post dismiss/re-render flows are unchanged.

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have run grunt build:images and committed the results, if my PR introduces or edits images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes Yoast/reserved-tasks#1196

[coveralls]

Coverage Report for CI Build 12426

Coverage decreased (-3.1%) to 53.516%

Details

• Coverage decreased (-3.1%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (3 of 5 lines covered, 60.0%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
admin/metabox/class-metabox.php	2	0	0.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	66117
Covered Lines:	35251
Line Coverage:	53.32%
Relevant Branches:	16977
Covered Branches:	9218
Branch Coverage:	54.3%
Branches in Coverage %:	Yes
Coverage Strength:	45497.03 hits per line

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💛 - Coveralls