feat(weave): add provider name to turns

[rgao-coreweave]

Summary

• Add optional provider_name to the Python Turn model.
• Thread it through Conversation.start_turn(), module-level start_turn(), Turn.record(), and log_turn().
• Emit it as gen_ai.provider.name on root invoke_agent spans in both streaming and batch flows.
• Add focused coverage for each public entry point.

Testing

• uv run --no-sync --group test python -m pytest tests/conversation/test_conversation_otel.py -q (179 passed)
• uvx ruff check weave/conversation/conversation.py tests/conversation/test_conversation_otel.py
• uvx ruff format --check weave/conversation/conversation.py tests/conversation/test_conversation_otel.py

Breaking changes

None. The new parameter and field are optional and default to an empty string.

Related

Prerequisite for #7642, which uses the typed parameter in the Claude Agent SDK integration.

[ro31337]

Seems the current standard has removed this label? open-telemetry/semantic-conventions-genai#289

Curious about what should gen_ai.provider.name represent on this internal invoke_agent span when one turn can use multiple model providers?

could setting it here, for example, conflict with the current OTel convention, which removed this attribute from internal agent spans?

[rgao-coreweave]

oh true, good findings!

From PR's design, the turn's provider name should propagate down to the children spans, and the children spans can override the provider name.

From current state, it wouldn't make sense to show this info at the turn level since one turn can clearly use multiple model providers.

I am okay dropping this PR as well!