Force corfu prefix length for chat #257

Job	Run time
test (ubuntu-latest, 28.2)	57s
test (ubuntu-latest, 30.2)	40s
test (ubuntu-latest, 29.4)	1m 6s
test (macos-latest, snapshot)	3m 36s
test (macos-latest, 28.2)	3m 53s
test (windows-latest, 29.4)	1h 15m 13s
test (ubuntu-latest, snapshot)	39s
test (windows-latest, 30.2)	1h 20m 28s
test (windows-latest, 28.2)	42m 46s
test (macos-latest, 29.4)	3m 6s
test (macos-latest, 30.2)	4m 14s
test (windows-latest, snapshot)	56m 4s
	4h 32m 42s

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