Make RaggedEnd and RaggedRange broadcast as scalars

[ChrisRackauckas-Claude]

Summary

• Adds Base.broadcastable(x::RaggedEnd) = Ref(x) and Base.broadcastable(x::RaggedRange) = Ref(x) to make these types broadcast as scalars
• Adds tests verifying the broadcast behavior

Problem

When using end indexing with ragged arrays in SymbolicIndexingInterface (e.g., sol[x[1], end]), broadcasting over RaggedEnd fails because Julia's broadcast machinery tries to collect it, which requires length:

MethodError: no method matching length(::RecursiveArrayTools.RaggedEnd)

Stack trace from ModelingToolkit CI:

[1] _similar_shape(itr::RecursiveArrayTools.RaggedEnd, ::Base.HasLength)
[2] _collect(cont::UnitRange{Int64}, itr::RecursiveArrayTools.RaggedEnd, ...)
[3] collect(itr::RecursiveArrayTools.RaggedEnd)
[4] broadcastable(x::RecursiveArrayTools.RaggedEnd)

Solution

By defining Base.broadcastable(x::RaggedEnd) = Ref(x), these types are now treated as scalars in broadcasting operations - similar to how Symbol, String, and other non-collection types work. This is semantically correct since RaggedEnd represents a single index value, not a collection.

Test plan

• Added tests verifying RaggedEnd and RaggedRange return Ref from broadcastable
• Added tests verifying identity.(ragged_idx) works without error
• All existing tests pass

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