feat(weave): first-class sdk feedback for agent spans + turns

[andrewtruong]

Summary

Enables attaching feedback to Agent-view traces after the fact via the SDK — the gap a user hit where classic call.feedback.add_reaction() / add_note() worked for traditional traces, but there was no clean path for agent spans/turns, and add_reaction on an agent ref wrote wandb.reaction.1, which the Agent view ignores.

• Adds client.get_agent_span_feedback(span_id) / get_agent_turn_feedback(trace_id) / get_agent_conversation_feedback(conversation_id), each returning a RefFeedbackQuery — no more hand-building weave:///.../agent_span/... URIs or raw FeedbackCreateReq.
• add_reaction() on an agent ref now writes wandb.agent_user_feedback with scorer_tags (the shape the Agent UI thumbs render). Call/object refs are unchanged (still wandb.reaction.1).

Heads-up for the reviewer: this is a git cherry-pick -x of the already-merged #7524. This branch is behind master and that patch is already present on master, so there is nothing net-new to land here. Opened as a draft for review/tracking only — close it if #7524 already covers the need.

Test plan

• pytest tests/trace/test_client_feedback.py --trace-server=fake → all green (no Docker locally; the fake backend mirrors ClickHouse at the interface level, and feat(weave): first-class sdk feedback for agent spans + turns #7524's CI validated both backends).
• ruff check on the changed files → clean.
• mypy 1.19.1 (pinned to the pre-commit rev) on the changed source → introduces no new type errors.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
weave/trace/feedback.py	88.88%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!