Issue #3586580: Fix summarize prompt translating text instead of preserving original language

Description

The default prompt for the ai_ckeditor_summarize plugin was translating the text instead of summarizing it in the original language. The phrase "in the same language" was ambiguous — models interpreted it as the interface/user language rather than the language of the input text.

The fix strengthens the instruction to explicitly forbid translation.

Closes #3586580

Testing instructions

1. Enable the ai_ckeditor module and configure a text format (e.g. Full HTML) with the Summarize plugin enabled under CKEditor AI settings.
2. Open a node edit form and paste text written in any language (e.g. English).
3. Select all the text, click the AI Summarize button, and confirm the summary is returned in the same language as the selected text — not in the interface language.

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

AI Compliance

Note

Check the one that best describes your usage, or leave all unchecked if AI was not significantly used.

• AI Assisted Code
  Mainly written by a human; AI used for autocomplete or partial generation under full human supervision.

• AI Generated Code
  Mainly generated by AI, reviewed and approved by a human before this MR was created.

• Vibe Coded
  Generated by AI and only functionally reviewed before this MR was created.