Draft: Support full Python slice syntax in expand_node_string using real instance/device counts

[harsh-sikhwal]

Summary

• Replaced the digit-only [start:stop] regex in expand_node_string (infragraph_service.py) with a slice parser supporting full Python slice syntax: [:], [:-1], [1:], [::2], and bare/negative single indices.
• Added a _resolve_slice_indices() helper that resolves open-ended or negative slice bounds using real counts pulled from:
  • _instance_data — for the leading instance name
  • _device_data[...].components — for chained nested component/device names
• Falls back to the old explicit non-negative start:stop behavior when counts aren't known yet; raises InfrastructureError when a slice can't be resolved without a known count.
• Fixed test_expand_node_string.py, which was calling a nonexistent _expand_node_string method (pre-existing bug — all 7 tests were erroring). Corrected to expand_node_string and added 2 new tests covering count-based slice resolution and the unresolvable-count error case.