Rename Vercel AI experimental_decide spans from evaluate to decide #25251

Follow getsentry/sentry-conventions#696: gen_ai.evaluate is deprecated in favor of gen_ai.decide.

ai 7.0.128 renamed experimental_evaluate to experimental_decide. The old name is a deprecated alias, and the tracing channel always publishes type: 'experimental_decide'. Vercel's own @ai-sdk/otel now names the span decide <model> with gen_ai.operation.name: decide. Our channel subscriber still makes evaluate <model> spans.

Target span

• Name: decide <model> (was evaluate <model>)
• Op: gen_ai.decide (was gen_ai.evaluate)
• gen_ai.operation.name: decide (was evaluate)
• No other change. gen_ai.input.messages / gen_ai.output.messages keep type: 'evaluation'.

Scope

• packages/server-utils/src/integrations/vercel-ai/vercel-ai-dc-subscriber.ts. Keep the experimental_evaluate channel type for ai 7.0.111 to 7.0.127.
• Update the tests: unit test test/integrations/vercel-ai/evaluate.test.ts; node-integration-tests suite vercelai/v6_v7; e2e app node-eve.
• Change the test scenarios to call experimental_decide. They still call the deprecated experimental_evaluate and will break when Vercel removes the alias.