Guard optional stream setEncoding

[StantonMatt]

Summary

• only call setEncoding when a child-process stream exposes that method
• preserves current behavior for Node streams while avoiding the Bun/Deno-compatible stream TypeError reported in Support Bun #305

Fixes #305.

Verification

• npm test (rerun passed with 43 passing; one earlier local run hit three Mocha 2s timeouts before passing on rerun)
• bun -e 'import { PythonShell } from "./index.ts"; await PythonShell.run("exit-code.py", { scriptPath: "test/python", stdio: ["ignore", "pipe", "pipe"] }); console.log("bun smoke passed");'
• git diff --check
• review-fix-loop clean