Does the prayer become invalid if the worshiper swallows his saliva mixed with gum blood, which was contaminated by the fabric of the prayer garment that entered his mouth, then returned to his mouth, knowing that he could not find another place to spit it out?

It seems you are experiencing some waswas (whispers/obsessive thoughts), so we advise you to ignore them and pay them no mind. When a part of your garment enters your mouth, you should remove it so that it does not affect the correctness of your recitation. Some scholars have explicitly stated that it is disliked to hold anything in the mouth if it does not affect the recitation, as mentioned in "Manh al-Jalil" by Muhammad Ulaysh al-Maliki. If you did not notice blood in your saliva before or during the prayer, then it is considered pure, and the bleeding that occurred yesterday does not make it impure. Therefore, spitting this saliva onto your clothes during prayer, or unintentionally swallowing a small amount of it, does not invalidate the prayer.