feat: add Video Understanding support to the Interactions API

Adds public support for server-initiated media processing steps:

- ProcessingCallStep and ProcessingResultStep
- ProcessingCallDelta and ProcessingResultDelta (streaming)
- VideoContent.name field
- Blob.display_name and FileData.display_name fields

PiperOrigin-RevId: 972042329
Change-Id: I6ead1d4d99daf71a9035e3763cad12c818079f0b