Normalize generation error notice messages

[ekamran]

What?

This PR normalizes error notice messages in the Excerpt Generation and Content Summarization experiments.

Both features were passing the caught JavaScript Error object directly to createErrorNotice(). They now pass a plain message string instead.

Why?

WordPress notices already identify these notices as errors. Passing the raw Error object caused the visible notice text to include a redundant Error: prefix.

For example, the Excerpt Generation notice previously rendered as:

Error: Content is required to generate an excerpt suggestion.

After this change, it renders as:

Content is required to generate an excerpt suggestion.

This makes the notices cleaner and more consistent with other generation features.

How?

• Extracts a string message from caught errors before creating the notice.
• Keeps string errors unchanged.
• Adds translated fallback messages for unexpected error values.
• Adds E2E coverage for both affected notice paths.

Testing Instructions

Excerpt Generation

1. Enable AI.
2. Enable the Excerpt Generation experiment.
3. Create a post with no content.
4. Click Generate excerpt.
5. Confirm the notice does not include the redundant Error: prefix.

Content Summarization

1. Enable AI.
2. Enable the Content Summarization experiment.
3. Create a post with enough content to enable Generate Summary.
4. Trigger a failed summary generation request.
5. Confirm the notice does not include the redundant Error: prefix.

Automated Testing

• npm run lint:js
• npm run typecheck
• npx wp-scripts test-playwright --config tests/e2e/playwright.config.ts tests/e2e/specs/experiments/excerpt-generation.spec.js tests/e2e/specs/experiments/content-summarization.spec.js --project=chromium

Focused E2E result: 12 passed.

Use of AI Tools

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.21%. Comparing base (19ae2eb) to head (7059e50).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #562   +/-   ##
==========================================
  Coverage      71.21%   71.21%           
  Complexity      1151     1151           
==========================================
  Files             67       67           
  Lines           5565     5565           
==========================================
  Hits            3963     3963           
  Misses          1602     1602           

Flag	Coverage Δ
unit	71.21% <ø> (ø)

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[ekamran]

Screenshots

Excerpt Generation - Before

Excerpt Generation - After

Content Summarization - Before

Content Summarization - After

[github-actions]

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

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