OpenViking 0.3.x requires X-OpenViking-Account and X-OpenViking-User headers for ROOT API key requests to tenant-scoped APIs. Previously the `!="default"` guard skipped these headers when account/user were the literal string "default", causing INVALID_ARGUMENT errors.
Remove the `!="default"` guard so headers are sent whenever account/user are truthy. Empty strings are still correctly skipped since `""` is falsy.
Update tests to reflect the new behavior:
- test_viking_client_headers_send_tenant_when_default: asserts "default" headers ARE present
- test_viking_client_headers_send_tenant_when_empty_falls_back_to_default: asserts "default" headers ARE present from constructor fallback
Based on #21775 by @happy5318