Fix featured image no-provider notice

[ekamran]

What?

Fix the Featured Image Generation no-provider behavior.

When no AI connector is configured, clicking Generate featured image now shows the shared connector-required notice with the Manage Connectors link.

Why?

Before this change, the featured image flow skipped the shared provider guard and continued into prompt generation. That caused a lower-level error such as:

Failed to generate prompt: Image prompt generation failed. Please ensure you have a connected provider that supports text generation.

This was inconsistent with other AI features.

How?

Use the existing ensureProvider() helper before starting featured image generation.

Testing Instructions

Automated checks run locally:

npm run lint:js -- src/features/image-generation/components/GenerateFeaturedImage.tsx tests/e2e/specs/experiments/no-provider-degradation.spec.js
npm run build
npm run typecheck
PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD=1 WP_BASE_URL=http://localhost:8889 npm run test:e2e -- tests/e2e/specs/experiments/no-provider-degradation.spec.js --grep "Featured Image Generation"

Manual verification:

1. Disable/remove AI connector credentials.
2. Enable Image Generation and Editing.
3. Open the post editor.
4. In the Featured Image panel, click Generate featured image.
5. Confirm the notice says: “This feature requires an AI Connector to function properly.”
6. Confirm the notice includes the Manage Connectors link.

Screenshots/Screencast

Before:

Shows the lower-level prompt generation error.

After:

Shows the shared connector-required notice with the Manage Connectors link.

Use of AI Tools

AI assistance: Yes
Tool(s): ChatGPT / Codex
Used for: Repository review, reproduction planning, implementation guidance, test updates, and local verification. I reviewed the changes, tested the behavior locally, and take responsibility for the final submission.

[github-actions]

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Co-authored-by: ekamran <ekamran@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.41%. Comparing base (396fcc3) to head (8a2c01c).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #576   +/-   ##
==========================================
  Coverage      71.41%   71.41%           
  Complexity      1158     1158           
==========================================
  Files             67       67           
  Lines           5584     5584           
==========================================
  Hits            3988     3988           
  Misses          1596     1596           

Flag	Coverage Δ
unit	71.41% <ø> (ø)

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