Add ThreadSafeEntityCache::snapshot() for consistent multi-query reads

[bburda]

Multiple independent cache reads in CapabilityGenerator can mix data from different generations. Add a snapshot() method that returns an immutable copy under a single lock for use cases that need consistency across multiple queries.

Problem:
CapabilityGenerator::generate_resource_collection() and generate_specific_entity() make multiple calls to node_.get_thread_safe_cache() for different query types (get_entity_data, get_app_operations, etc.). Between these calls, the cache could be updated by the discovery manager, resulting in a spec that mixes data from different entity cache generations.

Proposed fix:
Add a ThreadSafeEntityCache::snapshot() method that returns a lightweight immutable copy of the cache data under a single read lock. The CapabilityGenerator would use this snapshot for all queries within a single spec generation, ensuring consistency.