Surgical reapply of PR #37523 onto current main (the original branch
predates the SecretSource registry refactor). _build_safe_env() now
forwards env vars tagged in env_loader._SECRET_SOURCES — widened from
Bitwarden-only to any registered secret source (Bitwarden, 1Password,
plugin backends), since the provenance map is source-agnostic.
Explicit server env: config still wins; untagged secrets stay filtered.
Fixes#37499.