Make serialization backwards-compatible for collab server (#27626)

Sets up the collab server to accept the format of system message that
we'll introduce later for [prompt
caching](https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching).

Release Notes:

- N/A
85740ddaa4Richard Feldman committed on 3/27/2025, 10:20:10 PM· committed by GitHubparent6550a96
2 files changedLine totals unavailable