test(code-runtime-python): resolve the interpreter path in the shell wrappers

The review's portability warning: the six shell wrappers exec'd a bare
'python3', which /bin/sh resolves against its compiled-in default PATH while
the runtime spawns with env:{} — in environments where python3 is reachable
only through the caller's PATH (Nix, pyenv) every wrapper run would fail as
worker-exit. The wrappers now bake the resolved absolute interpreter path
(module-level resolvePythonBin, which the product spawn already uses), and
resolvePythonBin is exported for the tests.

packages/code-runtime/code-runtime-python/src/index.ts

@@ -393,7 +393,7 @@ export function readProcessStart(pid: number): string | undefined {
  * @param bin - the configured interpreter (absolute path or bare command).
  * @returns an absolute path when resolvable, else `bin` unchanged.
  */
-function resolvePythonBin(bin: string): string {
+export function resolvePythonBin(bin: string): string {
   if (isAbsolute(bin) || bin.includes('/')) return bin
   const path = process.env.PATH
   /* v8 ignore next -- PATH is set in every environment the runtime boots in; the guard is defensive. */