batches: Prevent batch spec with mounts for SSBC

Created by: Piszmog

Part of #31790 (closed).

Adds checks to prevent a batch spec with a mount in a step from being processed when running server-side batch changes.

• CreateBatchSpecFromRaw called from UI SSBC
• UpsertBatchSpecInput called from src batch remote command

Test plan

Go unit tests and manual testing.