fix(sdk): validate protocol SSE content type (#2718)

## Summary

A successful non-SSE response (for example, a JSON error page) could be
treated as a ready protocol stream, leaving consumers with an empty
event iterator. Validate the response Content-Type before resolving the
stream readiness promise, and propagate the failure to the event
iterator. Add regression coverage and a patch changeset.

## Verification

- `pnpm --filter @langchain/langgraph-sdk test
src/client/stream/transport/http.test.ts` (17 passed)
- `pnpm exec oxfmt --check libs/sdk/src/client/stream/transport/http.ts
libs/sdk/src/client/stream/transport/http.test.ts
.changeset/steady-sse-validation.md`
- `pnpm exec oxlint libs/sdk/src/client/stream/transport/http.ts
libs/sdk/src/client/stream/transport/http.test.ts`