feat: support pre-uploading files to a session before it starts #927

Job	Run time
detect-changes	8s
Frontend Unit Tests (Vitest)	1m 4s
Backend Unit Tests	42s
Ambient Runner Tests	0s
API Server Integration Tests	0s
Script Tests (model-discovery)	0s
CLI Build and Tests	0s
summary	2s
	1m 56s

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