fix: validate trace item object shapes

[snoopuppy582]

Fixes #86

Summary

• Added validation that messages, tool_calls, and events contain only object items.
• Included field name and item index in each validation error.
• Added unit coverage for invalid trace items and adapter error wrapping.

Tests

• uv run --python 3.12 --extra dev pytest tests/test_trace.py tests/test_adapters.py -q
• uv run --python 3.12 --extra dev pytest tests/test_cli.py tests/test_live_http.py -q
• uv run --python 3.12 --extra dev pytest -q
• git diff --check

AI-assisted contribution disclosure

• Tool used: OpenAI Codex.
• AI-assisted parts: small validation helper and focused unit tests.
• Human review: I checked the issue requirements, reviewed the diff, verified the errors include field/index details, and ran targeted plus full pytest suites locally.

Notes

• Existing trace fixture paths still pass through the full test suite.