Adding special handling for prompts when the given model does not support system prompts

[jakedahn]

Summary by cubic

Adds type-safe handling for models that don’t support system prompts by injecting the system text into the first message without breaking content types. Prevents “[object Object]” from appearing in prompts and keeps behavior consistent. Addresses Linear CT-885.

• Bug Fixes
  • Inserts system text as a proper text part when content is an array or object, or as a string when content is a string.
  • Unsets streamParams.system to avoid sending unsupported fields to the model.

[linear]

CT-885 `[object Object]` issue