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  passes on every unpublished target.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/rust/Cargo.lock

@@ -590,7 +590,7 @@ checksum = "1d87ecb2933e8aeadb3e3a02b828fed80a7528047e68b4f424523a0981a3a084"
 
 [[package]]
 name = "mxgw-cli"
-version = "0.1.2"
+version = "0.2.0"
 dependencies = [
  "clap",
  "futures-util",
@@ -1490,7 +1490,7 @@ dependencies = [
 
 [[package]]
 name = "zb-mom-ww-mxgateway-client"
-version = "0.1.2"
+version = "0.2.0"
 dependencies = [
  "futures-core",
  "futures-util",