Use UTF-8 safe functions when summarizing request messages

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Problem/Motivation

When using as logger_db and logger as backend for observability, saving fails because of wrongly encoded data is passed to the backend.

The promble comes from the method summarizeAiPayloadData which uses substr instead of mb_substr when truncating the payload for logging.

Steps to reproduce (required for bugs, but not feature requests)

Please provide information like AI modules enabled, which AI provider, browser, etc.

Install logger_db and logger. Perform an AI operation with long text. Observe the wrongly encoded truncated text which causes the logger to fail.

Proposed resolution

Replace string manipulation functions with UTF-8 safe alternatives.

AI usage (if applicable)

[ ] AI Assisted Issue
This issue was generated with AI assistance, but was reviewed and refined by the creator.

[ ] AI Assisted Code
This code was mainly generated by a human, with AI autocompleting or parts AI generated, but under full human supervision.

[ ] AI Generated Code
This code was mainly generated by an AI with human guidance, and reviewed, tested, and refined by a human.

[ ] Vibe Coded
This code was generated by an AI and has only been functionally tested.