fix(log): simplify JSON log serialization for invalid UTF-8

[joshtrichards]

Summary

Make LogDetails::logDetailsAsJSON() best-effort and deterministic by letting json_encode() handle malformed UTF-8 directly.

Instead of probing strings and attempting to guess their original encoding with mb_detect_encoding() / mb_convert_encoding(), this change:

• removes the per-value fallback loop
• uses JSON_INVALID_UTF8_SUBSTITUTE to sanitize invalid byte sequences
• keeps JSON_PARTIAL_OUTPUT_ON_ERROR as a defensive fallback for logging

Why

This code path is used for logging, so the priority is to emit usable JSON rather than fail or spend extra work trying to recover the original encoding (which can't be done reliably anyhow - so this is a more "honest" implementation).

The implementation is simpler and should be faster in both the common case (valid UTF-8) and the less common case (invalid UTF-8) by avoiding per-field encoding checks and encoding detection/conversion.

Notes

• Behavior for malformed UTF-8 changes from "best-effort repair" to deterministic substitution.
• Valid UTF-8 input should be unaffected.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI