Fix TraceToMessages KeyError on multi-turn rollouts

[Mr-Neutr0n]

Summary

Fixes #425.

• convert_to_openai_messages crashed with KeyError: 'role' when processing multi-turn agent rollouts where subsequent spans carry full conversation history. Some tracing instrumentations omit the role attribute for assistant turns that consist only of tool_calls.
• The fix infers role="assistant" when tool_calls is present but role is missing, and skips messages that have neither role nor tool_calls.
• Added defensive filtering for malformed tool call entries missing required keys (id, name, arguments).
• Added three regression tests covering: missing role inference, malformed tool call filtering, and role-less message skipping.

Test plan

• Existing tests test_trace_messages_adapter_builds_expected_conversations and test_trace_messages_adapter_handles_multiple_tool_calls continue to pass
• New test test_convert_to_openai_messages_infers_assistant_role_from_tool_calls reproduces the exact APO's TraceToMessages adapter fails with multi-turn agent rollouts (KeyError: 'role') #425 crash scenario and verifies the fix
• New test test_convert_to_openai_messages_skips_messages_missing_role validates graceful skip of role-less non-tool messages
• New test test_convert_to_openai_messages_skips_tool_calls_missing_keys validates filtering of incomplete tool call entries

[Mr-Neutr0n]

Friendly bump! Let me know if there's anything I should update or improve to help move this forward.