fix(CLI-45): standardize the CLI credential env var and fail fast on empty passwords

All five client CLIs now share one credential contract for `authenticate-user`:
flags `--password` / `--password-env` (Go: `-password` / `-password-env`) with
default env `MXGATEWAY_VERIFY_PASSWORD`, resolution flag-then-env, and a resolved
credential that is missing *or empty* is a usage error naming the flag and the
variable. The value is never echoed and never reaches the wire.

Go and Java previously sent an empty credential when the variable was unset,
turning a misconfigured environment into a real MXAccess authentication attempt.
Go now returns the guard error before dialing; Java throws a picocli
ParameterException instead of falling back to "". Python's `--password-env`
gained the canonical default and its UsageError names the resolved variable.
Rust treats an empty flag or env value as missing, with the resolution extracted
into a testable `resolve_verify_user_password`. .NET adopts the canonical flags
and keeps `--verify-user-password`, `--verify-user-password-env`, and
MXGATEWAY_VERIFY_USER_PASSWORD as deprecated aliases for one release.

Docs same commit: CrossLanguageSmokeMatrix.md gains the credential contract and
the per-CLI subcommand-coverage table (the documented-not-fixed half of the
finding); all five READMEs name the canonical variable and the fail-fast rule,
and the .NET README carries the deprecation note. Tracking flipped to Done in
both remediation registers with a change-log row.

No .proto changed; no generated code regenerated.

clients/python/tests/test_cli.py

@@ -752,7 +752,9 @@ def test_authenticate_user_reads_password_from_env(monkeypatch: pytest.MonkeyPat
     assert fake.last_request.command.authenticate_user.verify_user_password == "env-secret-pw"
 
 
-def test_authenticate_user_requires_a_password() -> None:
+def test_authenticate_user_requires_a_password(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("MXGATEWAY_VERIFY_PASSWORD", raising=False)
+
     result = CliRunner().invoke(
         main,
         [
@@ -770,6 +772,81 @@ def test_authenticate_user_requires_a_password() -> None:
 
     assert result.exit_code != 0
     assert "password is required" in result.output
+    # CLI-45: the usage error names the option and the canonical env var.
+    assert "--password" in result.output
+    assert "MXGATEWAY_VERIFY_PASSWORD" in result.output
+
+
+def test_authenticate_user_reads_password_from_canonical_default_env(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """CLI-45: --password-env defaults to MXGATEWAY_VERIFY_PASSWORD.
+
+    Exporting the canonical variable alone must satisfy the credential, with no
+    explicit --password-env flag — the same operator workflow as the other CLIs.
+    """
+    from zb_mom_ww_mxgateway.generated import mxaccess_gateway_pb2 as pb
+
+    reply = pb.MxCommandReply(
+        session_id="s1",
+        kind=pb.MX_COMMAND_KIND_AUTHENTICATE_USER,
+        protocol_status=pb.ProtocolStatus(code=pb.PROTOCOL_STATUS_CODE_OK),
+        authenticate_user=pb.AuthenticateUserReply(user_id=11),
+    )
+    fake = _FakeInvokeClient(reply)
+
+    async def fake_connect(options, **_kwargs):
+        return fake
+
+    monkeypatch.setattr(commands_module.GatewayClient, "connect", fake_connect)
+    monkeypatch.setenv("MXGATEWAY_VERIFY_PASSWORD", "canonical-env-pw")
+
+    result = CliRunner().invoke(
+        main,
+        [
+            "authenticate-user",
+            "--plaintext",
+            "--session-id",
+            "s1",
+            "--server-handle",
+            "3",
+            "--verify-user",
+            "operator",
+            "--json",
+        ],
+    )
+
+    assert result.exit_code == 0, result.output
+    assert json.loads(result.output)["userId"] == 11
+    assert "canonical-env-pw" not in result.output
+    assert fake.last_request.command.authenticate_user.verify_user_password == "canonical-env-pw"
+
+
+def test_authenticate_user_rejects_empty_password_value(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """CLI-45: an empty resolved credential fails fast, never reaching the wire."""
+    monkeypatch.setenv("MXGATEWAY_VERIFY_PASSWORD", "")
+
+    result = CliRunner().invoke(
+        main,
+        [
+            "authenticate-user",
+            "--plaintext",
+            "--session-id",
+            "s1",
+            "--server-handle",
+            "3",
+            "--verify-user",
+            "operator",
+            "--password",
+            "",
+            "--json",
+        ],
+    )
+
+    assert result.exit_code != 0
+    assert "password is required" in result.output
 
 
 def test_write_secured_command_does_not_echo_value_on_failure(