fix(cmd/librarian): log errors with error level (#2754)

Log level Error were lost in main when calling `log.Fatal` instead of
`slog.Error`. Errors returned appear as `INFO` messages instead of
`ERROR`.

This was likely an oversight switching to slog as default logger. In
internal/librarian/librarian.go, the `setupLogger` function calls
[slog.SetDefault()](https://github.com/googleapis/librarian/blob/13bcf280d28167d2c23ca40b61abf5e27e62972a/internal/librarian/librarian.go#L40).
This tells the slog package to become the default logger for the entire
application. A side effect of this is that slog also intercepts any
output from the standard log package with no level info.

Also added a validation step in e2e test on failure case for generate,
to verify error is reported with Error level.

Test run below to verify now it correctly reports error:
```
╰─$ go run ./cmd/librarian generate -push                                                                              
time=2025-11-03T12:56:21.109-05:00 level=INFO msg="Temporary working directory" dir=/tmp/librarian-4043056686
time=2025-11-03T12:56:21.110-05:00 level=INFO msg="repo not specified, using current working directory as repo root" path=/usr/local/google/home/zhumin/repos/librarian
time=2025-11-03T12:56:21.110-05:00 level=ERROR msg="librarian command failed" err="failed to validate config: no GitHub token supplied for push"
exit status 1
```

Fix #2686

Author: zhumin8

cmd/librarian/main.go

@@ -16,7 +16,7 @@ package main
 
 import (
 	"context"
-	"log"
+	"log/slog"
 	"os"
 
 	"github.com/googleapis/librarian/internal/librarian"
@@ -25,6 +25,7 @@ import (
 func main() {
 	ctx := context.Background()
 	if err := librarian.Run(ctx, os.Args[1:]...); err != nil {
-		log.Fatal(err)
+		slog.Error("librarian command failed", "err", err)
+		os.Exit(1)
 	}
 }

e2e_test.go

@@ -17,6 +17,7 @@
 package librarian
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"html"
@@ -135,14 +136,18 @@ func TestRunGenerate(t *testing.T) {
 			cmd := exec.Command("go", cmdArgs...)
 			cmd.Env = append(os.Environ(), fmt.Sprintf("%s=fake-token", config.LibrarianGithubToken))
 			cmd.Env = append(cmd.Env, "LIBRARIAN_GITHUB_BASE_URL="+server.URL)
-			cmd.Stderr = os.Stderr
+			var stderr bytes.Buffer
+			cmd.Stderr = &stderr
 			cmd.Stdout = os.Stdout
 			err := cmd.Run()
 			if test.wantErr {
 				if err == nil {
 					t.Fatalf("%s should fail", test.name)
 				}
 
+				if g, w := stderr.String(), "level=ERROR"; !strings.Contains(g, w) {
+					t.Errorf("got %q, wanted it to contain %q", stderr.String(), w)
+				}
 				// the exact message is not populated here, but we can check it's
 				// indeed an error returned from docker container.
 				if g, w := err.Error(), "exit status 1"; !strings.Contains(g, w) {