fix(anthropic): record finish_reasons for empty-content streams

[Solaris-star]

Summary

Streamed Anthropic responses that end with a valid stop_reason but an empty content array recorded neither gen_ai.response.finish_reasons nor gen_ai.output.messages.

Root cause

_process_response_item only copied message_delta.stop_reason onto already-accumulated content-block events. With no content blocks, events stayed empty, so set_streaming_response_attributes early-returned.

Change

• Store message-level stop_reason on complete_response during message_delta
• Pass the full complete_response into set_streaming_response_attributes (still accepts the legacy bare events list used by existing tests)
• Fallback: if events yield no finish reasons, use the message-level stop_reason
• When content is empty but stop_reason is present, still emit an empty assistant output.messages entry

Test plan

• test_streaming_empty_content_records_finish_reason
• test_streaming_process_response_item_stores_message_stop_reason
• Existing streaming tests remain compatible (bare events list still accepted)

Fixes #4362

Summary by CodeRabbit

• Bug Fixes
  • Improved streaming response tracking when responses contain no content blocks.
  • Finish reasons are now recorded consistently from message-level stop reasons.
  • Empty assistant responses now include the correct finish reason and an empty content structure.
• Tests
  • Added coverage for empty-content streaming responses and message-level stop reasons.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e0e85b06-5705-47d6-b527-014a5e7e69aa

📥 Commits

Reviewing files that changed from the base of the PR and between 93429cf and 44ca690.

📒 Files selected for processing (3)

• packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/span_utils.py
• packages/opentelemetry-instrumentation-anthropic/opentelemetry/instrumentation/anthropic/streaming.py
• packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py

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📝 Walkthrough

Walkthrough

Anthropic streaming accumulation now preserves message-level stop reasons. Span attribute generation accepts complete responses, records fallback finish reasons, and emits empty assistant messages for empty-content streams. Tests cover both behaviors.

Changes

Anthropic streaming completion attributes

Layer / File(s)	Summary
Message stop-reason propagation packages/opentelemetry-instrumentation-anthropic/.../streaming.py	message_delta stop reasons are stored on the complete response and copied to existing event finish reasons before span processing.
Streaming attribute emission and validation packages/opentelemetry-instrumentation-anthropic/.../span_utils.py, packages/opentelemetry-instrumentation-anthropic/tests/test_semconv_span_attrs.py	set_streaming_response_attributes accepts complete responses or event lists, uses message-level stop reasons as a fallback, emits empty assistant messages, and tests empty-content completion behavior.

Estimated code review effort: 3 (Moderate) | ~20 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title is concise and accurately describes the main fix for empty-content Anthropic streams.
Linked Issues check	✅ Passed	The PR implements #4362 by preserving message-level stop_reason and recording finish_reasons and output messages for empty-content streams.
Out of Scope Changes check	✅ Passed	The changes and tests stay focused on the Anthropic streaming bug and do not introduce unrelated scope.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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