Fix golangci-lint v2 compliance, make all passes clean

- Fix 314 errcheck violations (blank identifier for unrecoverable errors)
- Fix errorlint violation (errors.Is for io.EOF)
- Remove unused serveL7Route test helper
- Simplify Duration.Seconds() selectors in tests
- Remove unnecessary fmt.Sprintf in test
- Migrate exclusion rules from issues.exclusions to linters.exclusions (v2 schema)
- Add gosec test exclusions (G115, G304, G402, G705)
- Disable fieldalignment govet analyzer (optimization, not correctness)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/l7/prefixconn_test.go

@@ -13,27 +13,27 @@ func TestPrefixConnRead(t *testing.T) {
 	if err != nil {
 		t.Fatalf("listen: %v", err)
 	}
-	defer ln.Close()
+	defer func() { _ = ln.Close() }()
 
 	go func() {
 		conn, err := ln.Accept()
 		if err != nil {
 			return
 		}
-		defer conn.Close()
-		conn.Write([]byte("WORLD"))
+		defer func() { _ = conn.Close() }()
+		_, _ = conn.Write([]byte("WORLD"))
 	}()
 
 	conn, err := net.Dial("tcp", ln.Addr().String())
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	pc := NewPrefixConn(conn, []byte("HELLO"))
 
 	// Read all data: should get "HELLOWORLD".
-	conn.SetReadDeadline(time.Now().Add(5 * time.Second))
+	_ = conn.SetReadDeadline(time.Now().Add(5 * time.Second))
 	all, err := io.ReadAll(pc)
 	if err != nil {
 		t.Fatalf("ReadAll: %v", err)
@@ -48,22 +48,22 @@ func TestPrefixConnSmallReads(t *testing.T) {
 	if err != nil {
 		t.Fatalf("listen: %v", err)
 	}
-	defer ln.Close()
+	defer func() { _ = ln.Close() }()
 
 	go func() {
 		conn, err := ln.Accept()
 		if err != nil {
 			return
 		}
-		defer conn.Close()
-		conn.Write([]byte("CD"))
+		defer func() { _ = conn.Close() }()
+		_, _ = conn.Write([]byte("CD"))
 	}()
 
 	conn, err := net.Dial("tcp", ln.Addr().String())
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	pc := NewPrefixConn(conn, []byte("AB"))
 
@@ -79,7 +79,7 @@ func TestPrefixConnSmallReads(t *testing.T) {
 	}
 
 	// Now reads come from the underlying conn.
-	conn.SetReadDeadline(time.Now().Add(5 * time.Second))
+	_ = conn.SetReadDeadline(time.Now().Add(5 * time.Second))
 	rest, err := io.ReadAll(pc)
 	if err != nil {
 		t.Fatalf("ReadAll: %v", err)
@@ -94,25 +94,25 @@ func TestPrefixConnEmptyPrefix(t *testing.T) {
 	if err != nil {
 		t.Fatalf("listen: %v", err)
 	}
-	defer ln.Close()
+	defer func() { _ = ln.Close() }()
 
 	go func() {
 		conn, err := ln.Accept()
 		if err != nil {
 			return
 		}
-		defer conn.Close()
-		conn.Write([]byte("DATA"))
+		defer func() { _ = conn.Close() }()
+		_, _ = conn.Write([]byte("DATA"))
 	}()
 
 	conn, err := net.Dial("tcp", ln.Addr().String())
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	pc := NewPrefixConn(conn, nil)
-	conn.SetReadDeadline(time.Now().Add(5 * time.Second))
+	_ = conn.SetReadDeadline(time.Now().Add(5 * time.Second))
 	all, err := io.ReadAll(pc)
 	if err != nil {
 		t.Fatalf("ReadAll: %v", err)
@@ -127,12 +127,12 @@ func TestPrefixConnDelegates(t *testing.T) {
 	if err != nil {
 		t.Fatalf("listen: %v", err)
 	}
-	defer ln.Close()
+	defer func() { _ = ln.Close() }()
 
 	go func() {
 		conn, _ := ln.Accept()
 		if conn != nil {
-			conn.Close()
+			_ = conn.Close()
 		}
 	}()
 
@@ -140,7 +140,7 @@ func TestPrefixConnDelegates(t *testing.T) {
 	if err != nil {
 		t.Fatalf("dial: %v", err)
 	}
-	defer conn.Close()
+	defer func() { _ = conn.Close() }()
 
 	pc := NewPrefixConn(conn, []byte("X"))