[BUG] Missing full multibyte support in StatisticsWriterProcessor

[dkd-friedrich]

Describe the bug

StatisticsWriterProcessor truncates the given search keyword to fit into the prepared keywords column of tx_solr_statistics. This leads to errors if the string is cut off at the wrong place, e.g. Incorrect string value: '\xE0\xB8' for column tx_solr_statistics.keywords at row 1

By using mb_substr() instead of substr() this issue can be fixed.

We should:

• Use mb_substring()
• Catch and handle exceptions that occur on insert in the statistics table to prevent errors in frontend

To Reproduce
Steps to reproduce the behavior:

1. Activate statistics
2. Trigger search with a long multi-byte string, like นักท่องเที่ยวที่เข้ามาในประเทศออสเตรียมีจำนวนทั้งหมดกี่คน

Expected behavior

• Statistics entry can be written
• Frontend is not affected, even if statistic record couldn't be written

Used versions (please complete the following information):

• TYPO3 Version: 11, 12
• EXT:solr Version: 11.5, 12