Simplify Agents SDK trace spans

[Ankcorn]

Agents SDK traces currently emit many near-zero-duration implementation spans. That adds ingestion cost and hides the chat turn and model invocation users actually need.

Collapse startup, chat setup, and fiber lifecycle bookkeeping into the existing semantic hierarchy while keeping chat_turn, agent invocation, model calls, and result persistence visible.

Before

hibernatableWebSocket (50.18s)
├── agent_initialization (0ms)
├── durable_object_storage_get (0ms)
├── think_start (0ms)
└── chat_interaction (9.64s)
    ├── clear_previous_chat_state (0ms)
    ├── persist_chat_request_context (0ms)
    ├── load_chat_history (0ms)
    ├── persist_incoming_messages (0ms)
    ├── schedule_agent_alarm (0ms)
    ├── chat_turn (9.64s)
    │   ├── initialize_fiber (0ms)
    │   ├── persist_fiber_snapshot (0ms)
    │   ├── prepare_agent (0ms)
    │   ├── invoke_agent TestbedAgent (9.64s)
    │   │   └── chat @cf/moonshotai/kimi-k2.6 (9.64s)
    │   ├── persist_chat_result (9.64s)
    │   └── finalize_fiber (0ms)
    └── schedule_agent_alarm (0ms)

After

hibernatableWebSocket (50.18s)
├── agent_initialization (0ms)
└── chat_interaction (9.64s)
    ├── chat_interaction_setup (0ms)
    └── chat_turn (9.64s)
        ├── initialize_fiber (0ms)
        ├── invoke_agent TestbedAgent (9.64s)
        │   └── chat @cf/moonshotai/kimi-k2.6 (9.64s)
        └── persist_chat_result (9.64s)

[changeset-bot]

🦋 Changeset detected

Latest commit: 0a4c57e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
agents	Patch
@cloudflare/ai-chat	Patch
@cloudflare/think	Patch
@cloudflare/agent-think	Patch

Not sure what this means? Click here to learn what changesets are.

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[pkg-pr-new]

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agents

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@cloudflare/think

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commit: 0a4c57e

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.