[PORT] Add GuardContent to Python SystemContentBlock

[tummon]

Source SDK

strands-ts

Target SDK

strands-py

Related Files

strands-ts/src/types/messages.ts
  strands-ts/src/types/__tests__/messages.test.ts
  strands-ts/src/models/bedrock.ts
  strands-ts/src/models/__tests__/bedrock.test.ts

Confirmation

• The change is complete in the source SDK, and every related artifact is listed above (source, tests, and docs, if the change has any).