Add Container.validate() for cycle detection

[lesnik512]

Summary

• Add Container.validate() method that walks the provider graph via DFS and detects circular dependencies, raising RuntimeError with the full cycle path (e.g. "CycleA -> CycleB -> CycleA")
• Add validate=False parameter to Container.__init__ for opt-in validation at creation time
• Add get_dependencies() method to AbstractProvider and Factory to expose dependency edges without accessing internals
• Add __iter__ to ProvidersRegistry for clean graph traversal
• Zero runtime cost — validation only runs when explicitly requested

Test plan

• Cycle detection raises with correct path for A -> B -> A cycle
• validate=True flag triggers detection at container creation
• Valid dependency graphs pass without error
• All existing tests pass
• Linters and type checker pass