bun: test coverage is thin

* `bun test` reports 50 tests across 5 files. Cloudflare has 929, hono 241.
* No tests at all for `plugin.ts`, `client.ts`, `integrations/fetch.ts`.
* `dev-packages/bun-integration-tests` has 5 suites: `basic`, `fetch`, `hono-sdk`, `orchestrion-mysql`, `orchestrion-postgres`. Deno has 24.
* Bun tests run under `bun test`, not vitest, so `yarn test` at the repo root does not run them. Easy to forget locally.

**Work item.** Pick the three highest-value additions: `plugin.ts` (the only instrumentation path that actually works on Bun today), streaming responses once B3 lands, and one more real orchestrion integration suite that goes through `Bun.build` (redis or express).

**Prior art ***(related history only)*. [#18635]() (open) "Expand canary test coverage across all framework SDKs" is the nearest umbrella. No issue covers the Bun unit or integration test gap specifically.

Start by inspecting the existing Bun tests and the suites under dev-packages/bun-integration-tests, then run them with bun test rather than yarn test. Add coverage for plugin.ts, streaming responses once B3 lands, and one Bun.build-based orchestrion suite for redis or express. Done means the selected unit and integration paths are covered and pass under bun test.

索引モデルが issue の本文から書いたものです。