test_nos_server_config_1 crashes with a raw ValueError when SFTP_* env vars are unset, instead of failing clearly #216

tests/util/test_util_nos.py::TestUtilNos::test_nos_server_config_1 fails with an unhandled ValueError when the SFTP_* environment variables (SFTP_SERVER, SFTP_PORT, etc.) are not set -- e.g. in a dev environment where ~/dev/exports.sh has not been sourced.

Traceback (relevant frames):

csvpath/util/var_utility.py -- VarUtility.parse_var_value() / value_or_var_value():
if the config value is an all-caps token (e.g. "SFTP_PORT") and os.getenv() for it
returns None, the literal token string is returned unchanged, by design (this is
documented behavior, not itself a bug).

tests/util/test_util_nos.py:18-23 then builds:
    path = f"sftp://{server}:{port}/nos_test.txt"
with server/port still literally "SFTP_SERVER"/"SFTP_PORT", and passes it to
DataFileWriter.

.venv/.../smart_open/ssh.py:115 parse_uri() -> urllib.parse.urlsplit(uri).port
.venv/.../urllib/parse.py:182:
    raise ValueError(f"Port could not be cast to integer value as {port!r}")
ValueError: Port could not be cast to integer value as 'SFTP_PORT'

So an unsubstituted placeholder ("SFTP_PORT" the string, not a real port number) propagates all the way from config into a third-party library's URL parser several layers down, where it blows up with a confusing raw ValueError instead of failing clearly and close to the source.

David: reproduces in a dev VM that lacks the real SFTP_* env vars (exports.sh not sourced there); passes fine on the host where those are exported. So the underlying cause is an expected environment gap, not a functional bug in the port-vs-None fix from PR #215. Still, the failure mode itself is bad -- it should not be possible for an unresolved ALL_CAPS placeholder to reach a third-party URL parser and crash there. Worth either:

• having the SFTP-dependent tests skip cleanly when the real SFTP_* env vars are not present, rather than attempt a live connection with placeholder values, or
• having VarUtility/Config raise a clear, early, csvpath-level error when a required env-var-backed config value is still an unresolved placeholder at the point of use.

Found while re-verifying the full suite after merging PR #215 (SFTP/Nos port fix) and PR #214 (references-v3 function registry) together -- this failure is part of the long-standing "known 11-failure" local-environment baseline, unchanged by either of those PRs.