Resolve Client.Go-004..010 code-review findings

Client.Go-004: ran gofmt on alarms_test.go and galaxy_test.go; the tree is
now gofmt-clean.

Client.Go-005/009/010: migrated Dial/DialGalaxy off the deprecated
grpc.DialContext/WithBlock to grpc.NewClient via a shared dial helper, with
a DialTimeout-bounded readiness probe to keep fail-fast semantics; shared
callContext deadline arithmetic; updated the stale Dial doc comment. Test
harnesses use passthrough:///bufnet for the NewClient default-scheme change.

Client.Go-006: added GatewayError.Code() and an IsTransient(err) helper so
callers can classify transient gRPC failures.

Client.Go-007: newCorrelationID no longer returns an empty id when
crypto/rand fails — it falls back to a non-empty time+counter id.

Client.Go-008: added coverage_test.go for transport-credential resolution,
callContext deadline arithmetic, and native value/array edge kinds.

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

clients/go/mxgateway/session.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"io"
 	"sync"
+	"sync/atomic"
+	"time"
 
 	pb "gitea.dohertylan.com/dohertj2/mxaccessgw/clients/go/internal/generated"
 	"google.golang.org/grpc/codes"
@@ -547,10 +549,25 @@ func (s *Session) invokeCommand(ctx context.Context, command *MxCommand) (*MxCom
 	})
 }
 
+// correlationIDCounter backs the deterministic fallback id used when
+// crypto/rand is unavailable, so every command still carries a unique,
+// traceable correlation id.
+var correlationIDCounter atomic.Uint64
+
+// randRead is the entropy source for newCorrelationID. It is a package
+// variable solely so tests can simulate a crypto/rand failure.
+var randRead = rand.Read
+
+// newCorrelationID returns a unique correlation id for an MxCommandRequest.
+// It prefers 16 bytes of crypto/rand entropy; if rand.Read fails (rare) it
+// falls back to a "fallback-" prefixed id built from the current time and a
+// process-wide monotonic counter rather than returning an empty string, which
+// would leave the command untraceable in gateway logs.
 func newCorrelationID() string {
 	var buffer [16]byte
-	if _, err := rand.Read(buffer[:]); err != nil {
-		return ""
+	if _, err := randRead(buffer[:]); err != nil {
+		return fmt.Sprintf("fallback-%x-%x",
+			time.Now().UnixNano(), correlationIDCounter.Add(1))
 	}
 	return hex.EncodeToString(buffer[:])
 }