Pi-ai's `streamSimple`/`SimpleStreamOptions` has no per-request env
escape hatch; its Azure/AWS/GCP ADC providers read `process.env`
directly. The previous implementation wrote `extraEnv` into
`process.env` globally and relied on a `finally` after
`stream.result()` to clean up, so two concurrent `testCredential`
calls would clobber each other's env, expose the wrong credential
to in-flight streams, and on error paths fail to restore
previously-set values.
Wrap the env-mutating branch in a module-level promise chain so at
most one `extraEnv` test runs at a time, snapshot the prior value
of every key we overwrite, and restore (or delete when unset) in
`finally`. Tests that resolve to `extraEnv: undefined` bypass the
mutex entirely.
