bun default integrations diverge from Node's

`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.

Compare packages/bun/src/sdk.ts:48 with node/src/sdk/index.ts:getBaseDefaultIntegrations, then review the integration definitions for conversationIdIntegration and systemErrorIntegration. Add the conversation integration, decide the system-error behavior, and document the intentional child-process and worker-thread omissions with a reference to issue #20237; done means Bun's defaults and comments reflect these decisions.

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