fix: remove inline backtick fences from tool instruction prose (#11869)

fix: remove inline backtick fences from tool instruction prose (#11674)

The system prompt prefix and suffix contained raw triple-backtick
sequences in prose text, producing an odd number of fence markers.
This made it ambiguous for models to distinguish actual tool-call
code fences from inline mentions of the syntax. Remove inline fence
syntax from prose, relying on the adjacent examples to convey the
format.