What are the key factors to consider when selecting a valve extender for mountain biking, and how do the different materials and designs offered by various brands, such as Lezyne and Silca, impact performance, durability, and weight, and are there any specific models that stand out as being particularly well-suited for high-pressure applications or extreme terrain, and how do valve extender designs accommodate different types of wheels and tire configurations, such as tubeless and tube-type setups, and what role do valve extender lengths play in determining compatibility and ease of use, and are there any emerging trends or innovations in valve extender design that are worth considering when making a purchase, and how do the prices of different valve extender models compare to their performance and durability, and what are the potential drawbacks or limitations of using a valve extender, and how do they impact the overall performance and reliability of the wheel and tire system.