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/src/zb_mom_ww_mxgateway_cli/commands.py

@@ -32,6 +32,11 @@ logger = logging.getLogger(__name__)
 
 MAX_AGGREGATE_EVENTS = 10_000
 
+#: Canonical CLI credential environment variable, shared by every official client
+#: CLI (CLI-45) so one exported variable drives the same operator workflow in all
+#: five languages.
+DEFAULT_VERIFY_PASSWORD_ENV = "MXGATEWAY_VERIFY_PASSWORD"
+
 _BATCH_EOR = "__MXGW_BATCH_EOR__"
 
 
@@ -328,7 +333,8 @@ def write_secured(**kwargs: Any) -> None:
 )
 @click.option(
     "--password-env",
-    default=None,
+    default=DEFAULT_VERIFY_PASSWORD_ENV,
+    show_default=True,
     help="Environment variable holding the user password.",
 )
 @click.option("--correlation-id", default="", help="Client correlation id.")
@@ -834,17 +840,23 @@ async def _authenticate_user(**kwargs: Any) -> dict[str, Any]:
 def _resolve_password(kwargs: dict[str, Any]) -> str:
     """Resolve the authenticate-user password from --password or --password-env.
 
-    Prefers the explicit flag, then falls back to the named environment
-    variable. The resolved secret is never echoed; callers pass it into the
-    ``secrets`` redaction list so it cannot leak through a surfaced error.
+    Prefers the explicit flag, then falls back to the environment variable named
+    by ``--password-env`` (default :data:`DEFAULT_VERIFY_PASSWORD_ENV`). A missing
+    *or empty* value from either source is a usage error (CLI-45): the CLI never
+    sends a fabricated empty credential to the wire. The error names the option
+    and the variable only — the resolved secret is never echoed, and callers pass
+    it into the ``secrets`` redaction list so it cannot leak through a surfaced
+    error either.
     """
 
+    env_name = kwargs.get("password_env") or DEFAULT_VERIFY_PASSWORD_ENV
     password = kwargs.get("password")
     if not password:
-        env_name = kwargs.get("password_env")
-        password = os.environ.get(env_name) if env_name else None
+        password = os.environ.get(env_name)
     if not password:
-        raise click.UsageError("a password is required via --password or --password-env")
+        raise click.UsageError(
+            f"a password is required via --password or the {env_name} environment variable"
+        )
     return password