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/client_session_test.go

@@ -292,8 +292,11 @@ func newBufconnClient(t *testing.T, fake *fakeGatewayServer) (*Client, func()) {
 	dialer := func(ctx context.Context, _ string) (net.Conn, error) {
 		return listener.DialContext(ctx)
 	}
+	// grpc.NewClient defaults the target scheme to dns; the bufconn fake name
+	// is not DNS-resolvable, so use the passthrough scheme to hand the target
+	// straight to the context dialer.
 	client, err := Dial(context.Background(), Options{
-		Endpoint:  "bufnet",
+		Endpoint:  "passthrough:///bufnet",
 		APIKey:    "test-api-key",
 		Plaintext: true,
 		DialOptions: []grpc.DialOption{