fix: prefer W3C baggage conversation ID for span consistency

[handsomesix]

Summary

Fix inconsistent gen_ai.conversation.id values between LangGraph invoke_agent spans and other spans when using traceloop instrumentation.

The change introduces a shared conversation ID resolution mechanism that prioritizes:

1. Explicit conversation_id parameter
2. gen_ai.conversation.id from W3C baggage
3. None

This ensures all spans within the same trace use a consistent conversation ID.

Root Cause

invoke_agent spans relied on traceloop-derived values, which are sourced from thread_id.

At the same time, SAP Cloud SDK propagates the conversation identifier through the W3C baggage header. When the traceloop thread_id and baggage-provided conversation ID differ, invoke_agent spans receive a different gen_ai.conversation.id than the rest of the trace.

As a result, spans belonging to the same conversation can be associated with different conversation identifiers.

Changes

• Added _resolve_conversation_id() helper to centralize conversation ID resolution.
• Implemented the following priority order:
  • Explicit conversation_id parameter
  • W3C baggage gen_ai.conversation.id
  • None
• Applied the shared resolution logic to chat_span.
• Applied the shared resolution logic to invoke_agent_span.
• Ensured conversation ID handling is consistent across span types.

Tests

Added unit tests covering _resolve_conversation_id() behavior:

• Explicit conversation_id takes precedence.
• Falls back to W3C baggage value when no explicit parameter is provided.
• Returns None when baggage is not present.
• Returns None when baggage value is empty.

Risk

Low.

• Backward compatible.
• Existing explicit conversation_id behavior is unchanged.
• Only introduces baggage-based fallback when no explicit conversation ID is provided.
• Scope is limited to conversation ID resolution logic.

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