fix(trace): LLM_TTFT parent node pointing to wrong span + missing ERROR/CANCELLED status colors

[Dasooul03]

Summary

• Fix llm-first-packet (LLM_TTFT) node having wrong parentNodeId pointing to llm-stream-routing instead of the specific provider stream node
• Fix frontend trace detail page crashing in failover scenarios due to unmapped ERROR/CANCELLED statuses

Problem

Backend: LLM_TTFT parent node wrong

AbstractOpenAIStyleChatClient.doStreamChat() detached the span in a finally block, but RoutingLLMService.awaitFirstPacket() (annotated with @RagTraceNode("llm-first-packet")) executed after detach, causing the TTFT node to incorrectly point to llm-stream-routing as parent.

Expected trace tree:

llm-stream-routing
└── bailian-stream-chat (LLM_PROVIDER)
    └── llm-first-packet (LLM_TTFT)

Actual trace tree (before fix):

llm-stream-routing
├── bailian-stream-chat (LLM_PROVIDER)
└── llm-first-packet (LLM_TTFT)

Frontend: ERROR/CANCELLED statuses not mapped

STATUS_COLORS only mapped success/failed/running/default. In failover scenarios, provider nodes have CANCELLED or ERROR status, causing the page to crash.

Fix

Backend

• StreamCancellationHandle: added default void detach() {} for backward-compatible span lifecycle control
• AbstractOpenAIStyleChatClient.doStreamChat(): no longer detaches in finally; instead returns a handle that carries the detach responsibility
• RoutingLLMService.streamChat(): calls handle.detach() after awaitFirstPacket completes, wrapped in try-finally to guarantee detach on both success and failover paths

Frontend

• STATUS_COLORS: added error (red) and cancelled (gray) status mappings
• getStatusColors: added ?? STATUS_COLORS.default fallback to prevent crashes on unmapped statuses
• traceUtils.ts: added error/cancelled to statusLabel and statusBadgeVariant

Test Plan

• Normal chat: verify llm-first-packet parentNodeId points to the specific provider stream node
• Failover: simulate first-packet timeout triggering provider switch, verify correct trace tree structure after fallback
• Frontend trace detail page: verify ERROR/CANCELLED status nodes render correctly without crashing

Closes #35

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