SDK - Add bulk file retrieval method

[jenna-winkler]

Problem

The SDK's File.get() method requires a separate API call for each file. This creates N+1 query performance issues when retrieving multiple files.

Current usage in codebase:

• agentstack/agents/rag/src/rag/tools/files/utils.py

  Line 56 in 8ee2297

  # TODO: N+1 query issue, add bulk endpoint

Dependencies

Requires platform API bulk endpoint