Ensure JsonMapper defaults avoid nullable configuration access

[magicsunday]

M# Sweep — Verify compliance for this milestone

• composer ci:test

Summary

• ensure JsonMapper promotes a concrete JsonMapperConfig default and align its call sites
• promote collection doc block resolver collaborators to non-nullable promoted properties
• adopt rector-driven readability tweaks across mapping context, type resolver, and value strategies

Testing

• composer ci:test

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Codex Task