Validate mesh node distance

[saitharun7]

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[saitharun7]

This PR adds validation for mesh_node_distance in create_flat_singlescale_mesh_graph.

Previously, if mesh_node_distance was set to 0 or a negative value, the code would attempt to compute:

range_x / mesh_node_distance

This leads to a divide-by-zero warning and eventually an overflowerror when converting infinity to an integer.

Added a validation check:

if mesh_node_distance <= 0:
raise ValueError(
f"mesh_node_distance must be greater than 0, got {mesh_node_distance}"
)

This ensures invalid values are caught early with a clear error message.