Fix subscription cache memory configuration

[Caideyipi]

Summary

• cap the initial subscription poll response cache allocation at the configured memory limit while retaining the existing 20% startup target for larger limits
• validate subscription_cache_memory_usage_percentage as a finite value in [0, 1] and add localized error messages
• add regression tests for 5%, 10%, 20%, and 50% cache limits and invalid configuration values

Testing

• targeted CommonDescriptorSubscriptionCacheMemoryUsagePercentageTest (2 tests)
• targeted SubscriptionPollResponseCacheTest (2 tests)
• full reactor test-compile with the default English locale
• full reactor test-compile with with-zh-locale