feat: Use new eve instrumentation hooks

[Luca Forstner (lforst)]

Based on vercel/eve#1799.

Upgrade guide

The instrumentation-provider API requires eve@0.34.0 or newer and the Braintrust release containing this PR (expected braintrust@3.28.0). After that release is published, upgrade both packages:

pnpm add eve@^0.34.0 braintrust@^3.28.0

Enable instrumentation providers on the Eve agent:

export default defineAgent({
  experimental: { instrumentationProviders: true },
  // ...the rest of the agent configuration
});

Then export the Braintrust provider directly from instrumentation/braintrust.ts:

import { braintrustEveInstrumentation, initLogger } from "braintrust";

export default braintrustEveInstrumentation({
  metadata: {
    // Optional metadata added to every captured span.
  },
  setup: ({ agentName }) => {
    initLogger({ projectName: agentName });
  },
});

With the provider API, do not wrap the result in Eve defineInstrumentation, and do not pass Eve defineState. Remove the legacy braintrustEveHook configuration from root agents and subagents as well; the provider receives turn, model, and action lifecycle events for the entire trace and would otherwise duplicate instrumentation.

Existing Eve integrations can remain on the legacy API. They may continue exporting braintrustEveInstrumentation({ defineState, setup }) until they are ready to upgrade Eve and enable instrumentation providers.