Restore AbstractString key support for JSON.Object

[quinnj]

Fixes #449.

Summary

• simplify find_node_by_key so Object{String} lookup works with SubString{String} and other AbstractString keys again
• route Object{String} mutation and deletion through shared String-key helpers so string-like keys work consistently without recursive dispatch
• add a regression test that mirrors the issue repro with split(...)[2]

Testing

• julia --project=. test/object.jl
• julia --project=. -e 'using Pkg; Pkg.test()'

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.33%. Comparing base (f4fbb5a) to head (76bc682).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #450      +/-   ##
==========================================
+ Coverage   90.26%   90.33%   +0.07%     
==========================================
  Files           7        7              
  Lines        1366     1376      +10     
==========================================
+ Hits         1233     1243      +10     
  Misses        133      133              

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