fix(CLI-39): bump client versions off published 0.1.2; guard the publish pipeline

Converges all five clients on one version after four had drifted onto the
already-published 0.1.2/0.1.1 while their APIs kept changing underneath it:

- Rust Cargo.toml [package] + [workspace.package] -> 0.2.0 (CLIENT_VERSION
  already derives from CARGO_PKG_VERSION, no separate edit).
- Python pyproject.toml + version.py -> 0.2.0; new test asserts __version__
  matches pyproject.toml (closes the CLI-26 residual drift mode).
- Go mxgateway/version.go ClientVersion -> 0.2.0.
- .NET ZB.MOM.WW.MxGateway.Client.csproj <Version> -> 0.2.0.
- Java -> 0.2.1, not 0.2.0: the live Gitea Maven feed already had 0.2.0
  published (2026-06-26), before the CLI-37/38/40/41 conformance fixes
  changed the client's observable behavior, so reusing 0.2.0 would label
  two different APIs identically. Recorded as an exception in
  docs/ClientPackaging.md's new Versioning section.

Publish-pipeline guards:

- scripts/tag-go-module.ps1 implements the CLI-21 guard: after semver
  validation it refuses to tag unless clients/go/mxgateway/version.go's
  ClientVersion already matches the requested tag version.
- scripts/pack-clients.ps1 gains a Gitea package-registry collision guard
  wired into every per-language -Publish step; it aborts if the target
  name+version already exists rather than force-overwriting. Verified live
  against the real Gitea registry (credentials already present in this
  environment) — correctly refuses on every known-published artifact and
  passes on every unpublished target.

Docs updated in the same commit: docs/ClientPackaging.md (new Versioning
section), and the five client READMEs' stale 0.1.1/0.1.2 example versions.

No .proto changes. No publish performed.

clients/python/tests/test_cli.py

@@ -1,6 +1,8 @@
 """Tests for the Python CLI."""
 
 import json
+import tomllib
+from pathlib import Path
 
 import pytest
 from click.testing import CliRunner
@@ -12,6 +14,21 @@ from zb_mom_ww_mxgateway_cli.commands import main
 _BATCH_EOR = "__MXGW_BATCH_EOR__"
 
 
+def test_version_matches_pyproject_toml() -> None:
+    """`__version__` must track `pyproject.toml`'s `[project].version`.
+
+    The existing `version` command tests only assert self-consistency against
+    `__version__` (the two hardcoded literals could still drift from each
+    other without either test catching it — the CLI-26 residual drift mode).
+    This test pins `__version__` to the single source of truth instead.
+    """
+    pyproject_path = Path(__file__).resolve().parent.parent / "pyproject.toml"
+    with pyproject_path.open("rb") as handle:
+        pyproject = tomllib.load(handle)
+
+    assert __version__ == pyproject["project"]["version"]
+
+
 def test_require_certificate_validation_flag_flows_through_connect(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None: