fix: omit system parameter when None in AnthropicLlm

[voidborne-d]

Summary

Fixes #5318.

AnthropicLlm.generate_content_async passes system=None to the Anthropic API when no system instruction is set (e.g. when LlmEventSummarizer triggers event compaction). The Anthropic API rejects None — it expects either a str, a list of content blocks, or the parameter to be omitted entirely.

Changes

• Compute system_instruction once before the stream/non-stream branch: use the config value when present, otherwise NOT_GIVEN (which the Anthropic SDK treats as "omit this parameter").
• Pass the resolved value through to both the non-streaming messages.create call and the streaming helper _generate_content_streaming.
• No behavioral change when a system instruction is provided.

Reproduction

1. Configure an ADK app with a Claude model (via Vertex AI) and EventsCompactionConfig.
2. Run enough invocations to trigger compaction.
3. LlmEventSummarizer creates an LlmRequest with config.system_instruction = None.
4. Before this fix: 400 Bad Request — system: Input should be a valid list.
5. After this fix: compaction succeeds; the system parameter is omitted from the API call.

[adk-bot]

Response from ADK Triaging Agent

Hello @voidborne-d, thank you for submitting this pull request!

To ensure code quality and prevent future regressions, could you please add a unit test that covers the case where system_instruction is None? You can find more information about testing requirements in our contribution guidelines.

Thanks!

[voidborne-d]

Thanks for the review! I've added two unit tests in 7ccb1c9:

• test_generate_content_async_none_system_instruction_non_streaming — verifies system=NOT_GIVEN is passed (not None) when system_instruction is unset in the non-streaming path.
• test_generate_content_async_none_system_instruction_streaming — same verification for the streaming path.

Both follow the existing patterns in test_anthropic_llm.py and assert that the Anthropic API receives NOT_GIVEN instead of None, which prevents the 400 Bad Request from #5318.