Add assertion to `assertion_definitions.json` to make sure `µ` doesn't get garbled anywhere in the flow

[basiliskus]

DevEx/OpEx

Some clues on how to implement from slack:

• this is the sequence that char is converted to: https://www.google.com/search?q=EFBFBD
• This is the character that should (probably) not be contained in any output: https://www.fileformat.info/info/unicode/char/0fffd/index.htm

Tasks

• [ ] Something.

Additional Context

https://flexion.slack.com/archives/C055XTF22B0/p1727717357268009