Fix untranslated error strings

[yusufhay]

What?

Closes #181

Fixes several untranslated user-facing error strings in image-generation and summarization flows.

Why?

Issue #181 calls for an internationalization audit across user-facing surfaces. These helper functions still contained hard-coded English error messages that can surface in the UI, which makes the plugin less ready for translation.

How?

• wrap hard-coded image-generation error strings in __()
• localize the invalid-response fallback in summarization
• replace the string-interpolated prompt-generation failure with a translated sprintf() message
• keep the patch focused on concrete user-facing error paths

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: identifying likely untranslated strings and drafting the initial patch/PR text; final file changes were reviewed and committed by me.

Testing Instructions

1. Review the changed files and confirm the new fallback error strings use the ai text domain.
2. Verify the image-generation helpers now use translated fallback messages for invalid responses.
3. Verify summarization now uses a translated invalid-response fallback.

Screenshots or screencast

Not applicable.

Changelog Entry

Fixed - Localize several user-facing fallback error strings in image-generation and summarization flows.

[github-actions]

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Co-authored-by: yusufhay <yusufmudagal@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[dkotter]

@yusufhay Looks like there's a handful of conflicts here. Could you ensure your branch has the latest changes from develop and resolve those conflicts?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.16%. Comparing base (3ec6c5e) to head (f538b80).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #500      +/-   ##
=============================================
+ Coverage      69.15%   69.16%   +0.01%     
  Complexity       981      981              
=============================================
  Files             63       63              
  Lines           4649     4648       -1     
=============================================
  Hits            3215     3215              
+ Misses          1434     1433       -1     

Flag	Coverage Δ
unit	69.16% <ø> (+0.01%)	⬆️

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