Validate `proxy.config.cache.ram_cache.use_seen_filter`

[phongn]

In #13234, Copilot warned that sufficiently large values for proxy.config.cache.ram_cache.use_seen_filter could cause problems for the LRU cache algorithm if someone writes a large value by accident. We should fix Cache.cc so that the RAM cache parameters are properly validated.

[cmcfarlen]

Please do so, thank you.