Load-time sanitizer silently removed non-ASCII codepoints from any
env var ending in _API_KEY / _TOKEN / _SECRET / _KEY, turning
copy-paste artifacts (Unicode lookalikes, ZWSP, NBSP) into opaque
provider-side API_KEY_INVALID errors.
Warn once per key to stderr with the offending codepoints (U+XXXX)
and guidance to re-copy from the provider dashboard.