Add netbridge infrastructure detection scaffolding

[Lucacux]

Motivation

• Provide a way for netbridge to detect available NAT traversal operators and select an appropriate operator at runtime via DetectInfrastructure and a returned InfrastructureStatus.
• Make operator selection deterministic and testable by adding ordering and an empty container constructor to the operator registry (OperatorContainer / NewEmptyOperatorContainer).
• Introduce scaffolding for additional traversal methods (NAT-PMP, PCP, hole-punch) so protocol implementations can be added later using a common operator interface.
• Enforce configured allowed port ranges in the netbridge implementation to prevent forwarding/discovery outside policy via parseAllowedPorts/isPortAllowed.

Description

• Add DetectInfrastructure to the NetbridgeInterface, add InfrastructureStatus struct, and implement DetectInfrastructure in NetbridgeImpl to report enabled state, available operators, selected operator, and fallback availability.
• Extend operators.OperatorContainer with ordering support, NewEmptyOperatorContainer, ObtainFrom, AvailableNames, and an orderedOperators helper to support prioritized and deterministic operator selection; register new operator scaffolds in NewOperatorContainer.
• Add operator scaffolds operators/natpmp, operators/pcp, and operators/holepunch providing New... constructors and stubbed OperatorInterface methods that return clear "not implemented" errors.
• Implement obtainOperator in NetbridgeImpl, delegate public/local IP and port operations to the obtained operator when available, and add parseAllowedPorts/isPortAllowed logic to enforce config.GetNetbridge().AllowedPorts.

Testing

• No automated tests were executed as part of this change.
• Unit tests were added/updated in internal/infrastructure/netbridge/netbridge_test.go to cover DetectInfrastructure and use a fakeOperator, but they were not run.
• gofmt was applied to the modified files as part of the rollout.

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Codex Task