fix: restore errors.Is for flag.ErrHelp regressed by JSON-RPC merge (#3927)

#3736 changed `err == flag.ErrHelp` to `errors.Is(err, flag.ErrHelp)` in
`codegen/example/templates/client_endpoint_init.go.tpl` so the generated
example client passes `golangci-lint` with `errorlint` enabled.

That fix was later overwritten by #3734 (Add JSON-RPC 2.0 Transport
Support, commit 8931cbb), which appears to have rebased on a pre-#3736
state and silently reverted the line back to `err == flag.ErrHelp`.

This commit restores the `errors.Is` form and updates the affected
golden testdata files. No other source change is required — the
`errors` package is already in the import spec list for the generated
file (`codegen/example/example_client.go:41`).

Author: stakahashy

codegen/example/templates/client_endpoint_init.go.tpl

@@ -31,7 +31,7 @@
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())

codegen/example/testdata/client-no-server.golden

@@ -61,7 +61,7 @@ func main() {
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())

codegen/example/testdata/client-single-server-multiple-hosts-with-variables.golden

@@ -80,7 +80,7 @@ func main() {
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())

codegen/example/testdata/client-single-server-multiple-hosts.golden

@@ -61,7 +61,7 @@ func main() {
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())

codegen/example/testdata/client-single-server-single-host-with-variables.golden

@@ -77,7 +77,7 @@ func main() {
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())

codegen/example/testdata/client-single-server-single-host.golden

@@ -61,7 +61,7 @@ func main() {
 		}
 	}
 	if err != nil {
-		if err == flag.ErrHelp {
+		if errors.Is(err, flag.ErrHelp) {
 			os.Exit(0)
 		}
 		fmt.Fprintln(os.Stderr, err.Error())