Add a message streaming indicator in AI conversation #6049

Job	Run time
Check which packages changed	0s
AI bot Tests	39s
Boxel UI Tests	1m 29s
Lint	2m 3s
Boxel Motion Tests	1m 11s
Host Tests	14m 23s
Matrix Client Tests - Group 1	13m 38s
Matrix Client Tests - Group 2	12m 17s
Realm Server Tests	5m 22s
deploy-ai-bot	0s
Deploy boxel to staging	0s
	51m 2s

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