* Add system_instruction parameter to run_inference
Allow callers to provide a custom system instruction directly when calling
run_inference, without having to construct provider-specific context objects.
For OpenAI, the instruction is prepended as a system message (preserving
existing messages). For Anthropic, Google, and AWS Bedrock, it overrides the
single system field with a warning when an existing system instruction is
present in the context.
* Use system_instruction parameter in _generate_summary
Pass the summarization prompt via run_inference's system_instruction
parameter instead of embedding it as a system message in the context.
* Add changelog for #3968
