Write StructUtils.Selectors.List as a JSON array

[krynju]

StructUtils.Selectors.List <: AbstractVector, so JSON's AbstractVector{<:Pair}, AbstractVector{Any} and AbstractVector{Pair{K,Any}} methods are ambiguous with StructUtils' own arraylike(::StructStyle, ::List) / applyeach(::StructStyle, f, ::List). On master:

julia> using JSON, StructUtils; const List = StructUtils.Selectors.List

julia> JSON.json(JSON.lazy("""{"a":[{"b":1},{"b":2}]}""")[~, :b])   # a List{Any}
ERROR: MethodError: applyeach(::JSON.JSONWriteStyle, ::JSON.WriteClosure{…}, ::List{Any}) is ambiguous.

julia> JSON.json(List([:a => 1]))
ERROR: MethodError: arraylike(::JSON.JSONWriteStyle, ::List{Pair{Symbol, Int64}}) is ambiguous.

julia> JSON.json(List(Union{Int,Missing}[1, missing]))   # StructUtils' List applyeach skips `lower`
"[1,\"missing\"]"

This defines arraylike and applyeach for List on JSONStyle: a List is always written as a plain array of its lowered items. Vector{Any} items keep JSON's concrete-dispatch applyeach; anything else goes through StructUtils' generic AbstractArray method via invoke, which bypasses the AbstractVector{<:Pair} object methods on purpose, since List{<:Pair} is arraylike in StructUtils unlike Vector{<:Pair}.

After:

julia> JSON.json(JSON.lazy("""{"a":[{"b":1},{"b":2}]}""")[~, :b])
"[1,2]"

julia> JSON.json(List([:a => 1])), JSON.json(List(Union{Int,Missing}[1, missing])), JSON.json(List(Union{}[]))
("[{\"a\":1}]", "[1,null]", "[]")

Found while working on #500 (the Vector{Union{}} fix), whose AbstractVector{Union{}} method has the same List{Union{}} ambiguity; this PR resolves that one too, and the two PRs are independent. Selectors.List exists in StructUtils since 2.7.1, within the current compat. Full test suite passes locally except for the pre-existing juliac --trim segfault on Julia 1.12.7, which reproduces on unmodified master.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.59%. Comparing base (4f3dfdc) to head (942feb2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
+ Coverage   92.57%   92.59%   +0.01%     
==========================================
  Files           7        7              
  Lines        1927     1932       +5     
==========================================
+ Hits         1784     1789       +5     
  Misses        143      143              

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