Chicken stock and inosinate
Most raw meats contain high levels of glutamic acid bound in the proteins. Glutimates (the salts of the glutamic acid) are what impart the flavor of umami. Inosinate is one such salt. Proteins need to be broken down (cooked) to release the glutimates.  I have not been able to find a minimum temperature or time, but it is clear that cooking is necessary. However, you should also keep in mind that the umami flavor in a chicken stock comes from multiple sources, as vegetables also contain glutimates.

Cooking a chicken for 45 minutes in a pressure cooker will clearly result in over-cooked chicken. In terms of the release of glutimates, a pressure cooker does not appear to be more advantageous than a traditional pot. It certainly makes the production of stock quicker.

I would suggest three options: (a) save chicken scraps (bones, necks, wing tips, etc) for the specific purpose of making stock, and don't worry about over cooking the chicken, just make a good stock, (b) same as suggestion (a) except purchase less expensive legs (for example) to make stock, or (c) poach a chicken to your desired doneness and use the poaching liquid as a stock...or as the liquid portion of a more rich stock as a second step.