fix(weave): show contentless agent errors in conversations

[bcsherma]

What

Keep errored chat spans in the agent conversation projection even when they have no output text or reasoning. The API emits an empty assistant message carrying the span ID, timing, model, and ERROR status, so the existing conversation and turn timeline can show the failure.

Why

The conversation badge counts every ERROR span, but build_chat_messages previously dropped content spans without renderable content. A failed streaming chat call could therefore contribute to the badge while disappearing from the turn timeline and conversation view.

How

_emit_assistant_message still omits empty successful spans. It now emits a message when status_code == "ERROR", using the existing assistant-message schema and status rendering.

This does not require an API schema change or migration.

Testing

• uv run --group test python -m pytest tests/trace_server/test_genai_chat_view.py -q (56 passed, 1 expected failure)
• uvx ruff check weave/trace_server/agents/chat_view.py tests/trace_server/test_genai_chat_view.py
• uvx ruff format --check weave/trace_server/agents/chat_view.py tests/trace_server/test_genai_chat_view.py

Anything Else?

This PR is stacked on the exception-event detail fallback because the same contentless error span should expose its exception details when opened from the conversation.

[bcsherma]

• fix(weave): show contentless agent errors in conversations #7724 👈 (View in Graphite)
• fix(weave): surface OTel exception event details #7723
• master

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[codecov]

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✅ All modified and coverable lines are covered by tests.

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