Type-assert collected keys before sort! to avoid invalidations

[Beforerr]

When json! is inferred with x::Any (e.g. JSON.json!(::Vector{UInt8}, ::Int64, ::Any, ::WriteOptions{JSONWriteStyle}, ...)), the sort-keys path calls sort!(collect(keys(x)); by=...) with an Any argument. Since only a handful of sort! kwmethods exist, inference enumerates them and records a backedge on kwcall(::NamedTuple, ::typeof(sort!), ::Any), so any package that adds a sort! kwmethod invalidates json!.

Repro (Julia 1.13, JSON main):

using OrderedCollections, SnoopCompileCore
invs = @snoop_invalidations using JSON
using SnoopCompile
invalidation_trees(invs)

gives 5 trees (4 sort! methods from OrderedCollections, 1 from LibGit2), each invalidating 2 json! instances with 23 children total.

Fix: assert collect(keys(x))::Vector, so the call is sort!(::Vector; by), which no OrderedCollections/LibGit2 method matches. With this change the repro above reports no invalidation trees. Tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.57%. Comparing base (4f3dfdc) to head (f4f6df4).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #501   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
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  Files           7        7           
  Lines        1927     1927           
=======================================
  Hits         1784     1784           
  Misses        143      143           

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