refactor(headers): centralize header logic using EnsureHeader utility

- Introduced `EnsureHeader` in `internal/misc/header_utils.go` to streamline header setting across executors.
- Updated Codex, Claude, and Gemini executors to utilize `EnsureHeader` for consistent header application.
- Incorporated Gin context headers (if available) into request header manipulation for better integration.

internal/misc/header_utils.go

@@ -0,0 +1,24 @@
+package misc
+
+import (
+	"net/http"
+	"strings"
+)
+
+func EnsureHeader(target http.Header, source http.Header, key, defaultValue string) {
+	if target == nil {
+		return
+	}
+	if source != nil {
+		if val := strings.TrimSpace(source.Get(key)); val != "" {
+			target.Set(key, val)
+			return
+		}
+	}
+	if strings.TrimSpace(target.Get(key)) != "" {
+		return
+	}
+	if val := strings.TrimSpace(defaultValue); val != "" {
+		target.Set(key, val)
+	}
+}