Resolve Client.Go-001 code-review finding

MxAccessError.Unwrap returned e.Command directly; on the HRESULT-only path
Command is a nil *CommandError, so Unwrap returned a non-nil error wrapping
a typed nil and errors.As bound a nil *CommandError. Unwrap now returns an
untyped nil when Command is nil. Added errors_test.go regression coverage
for the HRESULT-only and populated-Command paths.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

code-reviews/Client.Go/findings.md

@@ -7,7 +7,7 @@
 | Review date | 2026-05-18 |
 | Commit reviewed | `3cc53a8` |
 | Status | Reviewed |
-| Open findings | 10 |
+| Open findings | 9 |
 
 ## Checklist coverage
 
@@ -33,13 +33,13 @@
 | Severity | High |
 | Category | Correctness & logic bugs |
 | Location | `clients/go/mxgateway/errors.go:88-93`, `clients/go/mxgateway/errors.go:117-128` |
-| Status | Open |
+| Status | Resolved |
 
 **Description:** `MxAccessError.Unwrap` returns `e.Command` directly. `EnsureMxAccessSuccess` constructs `&MxAccessError{Reply: reply}` with `Command` left nil (the HRESULT / failing-`MxStatusProxy` path). When `Command` is a nil `*CommandError`, `Unwrap()` returns a non-nil `error` interface wrapping a nil pointer. Consequently `errors.As(err, &ce)` for `*CommandError` returns `true` while setting `ce` to nil — a caller writing the idiomatic `if errors.As(err, &commandErr) { use commandErr.Status }` nil-dereferences and panics. Verified empirically; the existing test only exercises the populated-`Command` path.
 
 **Recommendation:** Make `Unwrap` return an untyped nil when `Command` is nil: `if e == nil || e.Command == nil { return nil }; return e.Command`. Add a test for the HRESULT-only `MxAccessError` asserting `errors.As(err, &ce)` is `false`.
 
-**Resolution:** _(open)_
+**Resolution:** Resolved 2026-05-18: `MxAccessError.Unwrap` now returns an untyped nil when `Command` is nil, so `errors.As` no longer binds a typed-nil `*CommandError`; added `errors_test.go` regression coverage for the HRESULT-only and populated-`Command` paths.
 
 ### Client.Go-002