fix: resolve all golangci-lint v2 failures

Production fixes:
- cmd/server/main.go: refactor to run() helper to eliminate exitAfterDefer
  (os.Exit in main() no longer bypasses deferred s.Close)
- internal/cast/client.go: use _ = resp.Body.Close() (errcheck)
- internal/ghl/api.go: wrap both defers as func(){ _ = resp.Body.Close() }()

Test fixes:
- internal/cast/client_test.go: replace err.(*CastAPIError) type assertions
  with errors.As (errorlint)

Config:
- .golangci.yml: use explicit path regex .*_test\\.go and add errorlint
  to test-file exclusions

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.golangci.yml

@@ -18,7 +18,8 @@ linters:
 issues:
   exclusions:
     rules:
-      - path: _test\.go
+      - path: ".*_test\\.go"
         linters:
           - errcheck
           - gocritic
+          - errorlint

cmd/server/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
 	"net/http"
 	"os"
@@ -20,21 +21,25 @@ import (
 
 func main() {
 	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{Level: slog.LevelInfo})))
+	if err := run(); err != nil {
+		slog.Error("fatal error", "err", err)
+		os.Exit(1)
+	}
+}
 
+func run() error {
 	cfg, err := config.Load()
 	if err != nil {
-		slog.Error("config error", "err", err)
-		os.Exit(1)
+		return fmt.Errorf("config: %w", err)
 	}
 
 	ctx := context.Background()
 
 	s, err := store.NewStore(ctx, cfg.MongoURI)
 	if err != nil {
-		slog.Error("failed to connect to mongodb", "err", err)
-		os.Exit(1)
+		return fmt.Errorf("mongodb: %w", err)
 	}
-	defer s.Close(ctx)
+	defer func() { _ = s.Close(ctx) }()
 
 	castClient := cast.NewClient(cfg.CastAPIURL, cfg.CastAPIKey, cfg.CastSenderID)
 	ghlAPI := ghl.NewAPIClient()
@@ -42,8 +47,7 @@ func main() {
 
 	webhookHandler, err := ghl.NewWebhookHandler(cfg.GHLWebhookPublicKey, castClient, ghlAPI, oauthHandler, s)
 	if err != nil {
-		slog.Error("failed to initialize webhook handler", "err", err)
-		os.Exit(1)
+		return fmt.Errorf("webhook handler: %w", err)
 	}
 
 	r := chi.NewRouter()
@@ -77,9 +81,9 @@ func main() {
 
 	slog.Info("cast-ghl-provider started", "port", cfg.Port, "base_url", cfg.BaseURL)
 	if err := srv.ListenAndServe(); err != http.ErrServerClosed {
-		slog.Error("server error", "err", err)
-		os.Exit(1)
+		return fmt.Errorf("server: %w", err)
 	}
+	return nil
 }
 
 func healthCheck(w http.ResponseWriter, r *http.Request) {

internal/cast/client.go

@@ -60,7 +60,7 @@ func (c *Client) SendSMS(ctx context.Context, to, message string) (*SendResponse
 				}
 			}
 			slog.Warn("cast api rate limited, retrying", "attempt", attempt+1, "wait", wait)
-			resp.Body.Close()
+			_ = resp.Body.Close()
 			select {
 			case <-ctx.Done():
 				return nil, ctx.Err()
@@ -70,7 +70,7 @@ func (c *Client) SendSMS(ctx context.Context, to, message string) (*SendResponse
 		}
 
 		data, err := io.ReadAll(resp.Body)
-		resp.Body.Close()
+		_ = resp.Body.Close()
 		if err != nil {
 			return nil, err
 		}

internal/cast/client_test.go

@@ -3,6 +3,7 @@ package cast
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"net/http"
 	"net/http/httptest"
 	"sync/atomic"
@@ -52,8 +53,8 @@ func TestSendSMS_APIError(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error, got nil")
 	}
-	castErr, ok := err.(*CastAPIError)
-	if !ok {
+	var castErr *CastAPIError
+	if !errors.As(err, &castErr) {
 		t.Fatalf("expected CastAPIError, got %T", err)
 	}
 	if castErr.StatusCode != http.StatusPaymentRequired {
@@ -73,8 +74,8 @@ func TestSendSMS_SuccessFalseInBody(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error, got nil")
 	}
-	castErr, ok := err.(*CastAPIError)
-	if !ok {
+	var castErr *CastAPIError
+	if !errors.As(err, &castErr) {
 		t.Fatalf("expected CastAPIError, got %T", err)
 	}
 	if castErr.APIError != "invalid number" {
@@ -133,8 +134,8 @@ func TestSendSMS_Unauthorized(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected error for 401, got nil")
 	}
-	castErr, ok := err.(*CastAPIError)
-	if !ok {
+	var castErr *CastAPIError
+	if !errors.As(err, &castErr) {
 		t.Fatalf("expected CastAPIError, got %T: %v", err, err)
 	}
 	if castErr.StatusCode != http.StatusUnauthorized {

internal/ghl/api.go

@@ -44,7 +44,7 @@ func (c *APIClient) UpdateMessageStatus(ctx context.Context, accessToken, messag
 	if err != nil {
 		return err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 	respBody, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return fmt.Errorf("ghl update status: failed to read response body: %w", err)
@@ -75,7 +75,7 @@ func (c *APIClient) PostInboundMessage(ctx context.Context, accessToken string,
 	if err != nil {
 		return nil, err
 	}
-	defer resp.Body.Close()
+	defer func() { _ = resp.Body.Close() }()
 	respBody, err := io.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err