fix(observability): bound websocket trace spans

[Ankcorn]

Why

A canceled or disconnected hibernatable WebSocket turn can end its Worker invocation before JavaScript stream finalizers run. This left exactly five custom spans open: chat_interaction, chat_turn, invoke_agent, persist_chat_result, and the active chat span.

What

The tracer now owns bounded span lifetimes. Call sites declaratively choose one of two policies:

• finish after a matching child span opens
• finish when a callback hands off asynchronous work

There are no finish() callbacks to thread through the chat lifecycle. The tracer keeps the finished span's async context active, preserving the existing hierarchy:

chat_interaction
└── chat_turn
    ├── invoke_agent
    │   └── chat
    └── persist_chat_result

These policies apply only to ws-chat; RPC and alarm turns retain full-stream span lifetimes and attributes.

Evidence

A local Service Trace Worker reproduced both explicit cancellation and hard disconnects:

before: 65 SpanOpen / 60 SpanClose
 after: 65 SpanOpen / 65 SpanClose

Completed WebSocket turns remain balanced and raw parent IDs remain unchanged. Agents tracing tests and all AI Chat (646) and Think (910) Worker tests pass.

Stacked on cloudflare#1860.