refactor: improve error handling and code quality

- Handle errors in promptInput instead of ignoring them
- Improve promptSelect to provide feedback on invalid input and re-prompt
- Use sentinel errors (ErrAuthorizationPending, ErrSlowDown) instead of
  string-based error checking with strings.Contains
- Move hardcoded x-amz-user-agent header to idcAmzUserAgent constant

Addresses code review feedback from Gemini Code Assist.

internal/auth/kiro/sso_oidc.go

@@ -8,6 +8,7 @@ import (
 	"crypto/sha256"
 	"encoding/base64"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"html"
 	"io"
@@ -35,13 +36,22 @@ const (
 
 	// Polling interval
 	pollInterval = 5 * time.Second
-	
+
 	// Authorization code flow callback
 	authCodeCallbackPath = "/oauth/callback"
 	authCodeCallbackPort = 19877
-	
+
 	// User-Agent to match official Kiro IDE
 	kiroUserAgent = "KiroIDE"
+
+	// IDC token refresh headers (matching Kiro IDE behavior)
+	idcAmzUserAgent = "aws-sdk-js/3.738.0 ua/2.1 os/other lang/js md/browser#unknown_unknown api/sso-oidc#3.738.0 m/E KiroIDE"
+)
+
+// Sentinel errors for OIDC token polling
+var (
+	ErrAuthorizationPending = errors.New("authorization_pending")
+	ErrSlowDown             = errors.New("slow_down")
 )
 
 // SSOOIDCClient handles AWS SSO OIDC authentication.
@@ -104,7 +114,11 @@ func promptInput(prompt, defaultValue string) string {
 	} else {
 		fmt.Printf("%s: ", prompt)
 	}
-	input, _ := reader.ReadString('\n')
+	input, err := reader.ReadString('\n')
+	if err != nil {
+		log.Warnf("Error reading input: %v", err)
+		return defaultValue
+	}
 	input = strings.TrimSpace(input)
 	if input == "" {
 		return defaultValue
@@ -112,24 +126,32 @@ func promptInput(prompt, defaultValue string) string {
 	return input
 }
 
-// promptSelect prompts the user to select from options using arrow keys or number input.
+// promptSelect prompts the user to select from options using number input.
 func promptSelect(prompt string, options []string) int {
-	fmt.Println(prompt)
-	for i, opt := range options {
-		fmt.Printf("  %d) %s\n", i+1, opt)
-	}
-	fmt.Print("Enter selection (1-", len(options), "): ")
-
 	reader := bufio.NewReader(os.Stdin)
-	input, _ := reader.ReadString('\n')
-	input = strings.TrimSpace(input)
 
-	// Parse the selection
-	var selection int
-	if _, err := fmt.Sscanf(input, "%d", &selection); err != nil || selection < 1 || selection > len(options) {
-		return 0 // Default to first option
+	for {
+		fmt.Println(prompt)
+		for i, opt := range options {
+			fmt.Printf("  %d) %s\n", i+1, opt)
+		}
+		fmt.Printf("Enter selection (1-%d): ", len(options))
+
+		input, err := reader.ReadString('\n')
+		if err != nil {
+			log.Warnf("Error reading input: %v", err)
+			return 0 // Default to first option on error
+		}
+		input = strings.TrimSpace(input)
+
+		// Parse the selection
+		var selection int
+		if _, err := fmt.Sscanf(input, "%d", &selection); err != nil || selection < 1 || selection > len(options) {
+			fmt.Printf("Invalid selection '%s'. Please enter a number between 1 and %d.\n\n", input, len(options))
+			continue
+		}
+		return selection - 1
 	}
-	return selection - 1
 }
 
 // RegisterClientWithRegion registers a new OIDC client with AWS using a specific region.
@@ -266,10 +288,10 @@ func (c *SSOOIDCClient) CreateTokenWithRegion(ctx context.Context, clientID, cli
 		}
 		if json.Unmarshal(respBody, &errResp) == nil {
 			if errResp.Error == "authorization_pending" {
-				return nil, fmt.Errorf("authorization_pending")
+				return nil, ErrAuthorizationPending
 			}
 			if errResp.Error == "slow_down" {
-				return nil, fmt.Errorf("slow_down")
+				return nil, ErrSlowDown
 			}
 		}
 		log.Debugf("create token failed: %s", string(respBody))
@@ -315,7 +337,7 @@ func (c *SSOOIDCClient) RefreshTokenWithRegion(ctx context.Context, clientID, cl
 	req.Header.Set("Content-Type", "application/json")
 	req.Header.Set("Host", fmt.Sprintf("oidc.%s.amazonaws.com", region))
 	req.Header.Set("Connection", "keep-alive")
-	req.Header.Set("x-amz-user-agent", "aws-sdk-js/3.738.0 ua/2.1 os/other lang/js md/browser#unknown_unknown api/sso-oidc#3.738.0 m/E KiroIDE")
+	req.Header.Set("x-amz-user-agent", idcAmzUserAgent)
 	req.Header.Set("Accept", "*/*")
 	req.Header.Set("Accept-Language", "*")
 	req.Header.Set("sec-fetch-mode", "cors")
@@ -426,12 +448,11 @@ func (c *SSOOIDCClient) LoginWithIDC(ctx context.Context, startURL, region strin
 		case <-time.After(interval):
 			tokenResp, err := c.CreateTokenWithRegion(ctx, regResp.ClientID, regResp.ClientSecret, authResp.DeviceCode, region)
 			if err != nil {
-				errStr := err.Error()
-				if strings.Contains(errStr, "authorization_pending") {
+				if errors.Is(err, ErrAuthorizationPending) {
 					fmt.Print(".")
 					continue
 				}
-				if strings.Contains(errStr, "slow_down") {
+				if errors.Is(err, ErrSlowDown) {
 					interval += 5 * time.Second
 					continue
 				}
@@ -639,10 +660,10 @@ func (c *SSOOIDCClient) CreateToken(ctx context.Context, clientID, clientSecret,
 		}
 		if json.Unmarshal(respBody, &errResp) == nil {
 			if errResp.Error == "authorization_pending" {
-				return nil, fmt.Errorf("authorization_pending")
+				return nil, ErrAuthorizationPending
 			}
 			if errResp.Error == "slow_down" {
-				return nil, fmt.Errorf("slow_down")
+				return nil, ErrSlowDown
 			}
 		}
 		log.Debugf("create token failed: %s", string(respBody))
@@ -787,12 +808,11 @@ func (c *SSOOIDCClient) LoginWithBuilderID(ctx context.Context) (*KiroTokenData,
 		case <-time.After(interval):
 			tokenResp, err := c.CreateToken(ctx, regResp.ClientID, regResp.ClientSecret, authResp.DeviceCode)
 			if err != nil {
-				errStr := err.Error()
-				if strings.Contains(errStr, "authorization_pending") {
+				if errors.Is(err, ErrAuthorizationPending) {
 					fmt.Print(".")
 					continue
 				}
-				if strings.Contains(errStr, "slow_down") {
+				if errors.Is(err, ErrSlowDown) {
 					interval += 5 * time.Second
 					continue
 				}