ci: verify the client SDK on Deno and Bun

[satwiksps]

Goal

Add native compatibility checks for @streamweld/client on Deno and Bun. The package intentionally uses no Node-only APIs, but today CI verifies it only through Node/Vitest.

Scope

• Add pinned Deno and Bun setup steps or jobs to .github/workflows/ci.yml.
• Add a small smoke test that imports the built ESM package and consumes a deterministic SSE response.
• Keep the existing Node/Vitest matrix unchanged.
• Document the tested runtime versions in packages/client/README.md.

Acceptance Criteria

• The smoke test runs under both Deno and Bun.
• CI fails if the SDK imports a Node-only API.
• Runtime versions are pinned for reproducibility.
• pnpm typecheck, pnpm test, and pnpm build remain green.

Contributor Notes

The existing deterministic fixtures in packages/client/src/client.test.ts show the expected SSE event shapes. Please keep the new test focused on runtime compatibility rather than duplicating the full Vitest suite.