bun default integrations diverge from Node's

[isaacs]

packages/bun/src/sdk.ts:48 omits, relative to node/src/sdk/index.ts:getBaseDefaultIntegrations:

Integration	Note
conversationIdIntegration	pure core JS, no Node API. Cloudflare and vercel-edge both include it. Without it, setConversationId has no effect on gen_ai.* spans under Bun.
systemErrorIntegration	needs a check
childProcessIntegration	relies on child_process diagnostics channels Bun does not emit (see B1), so probably correct to omit, but say so in a comment
workerThreadsIntegration	same

Work item. Add conversationIdIntegration() to Bun's defaults. For each remaining omission, either add it or leave a one-line comment saying which Bun gap makes it a no-op.

**Prior art **(tracked). #20237 (closed, not planned) already settled the childProcessIntegration and workerThreadsIntegration question, and documents the same root cause as B1: Bun implements the node:diagnostics_channel API but never publishes to the child_process or worker_threads channels. Omitting those two is correct; cite that issue in the comment. Nothing covers conversationIdIntegration or systemErrorIntegration on Bun.

[linear-code]

JS-3513