Replace Jason with Elixir's built-in JSON module

[Sim-hu]

Summary

Replace the external jason dependency with Elixir 1.18's built-in JSON module, as proposed in #666.

Changes

• 29 files changed, net reduction of 24 lines
• All Jason.decode!/Jason.decode calls replaced with JSON.decode!/JSON.decode
• All Jason.encode_to_iodata! calls replaced with JSON.encode_to_iodata!
• All @derive Jason.Encoder replaced with @derive JSON.Encoder
• Custom Jason.Encoder protocol implementations (embed structs) rewritten for JSON.Encoder
• keys: :atoms option replaced with Util.safe_atom_map/1 (already used elsewhere in the codebase for atom key conversion)
• Minimum Elixir version bumped to ~> 1.18
• jason removed from deps in mix.exs

Notes

• Chose Elixir's JSON module over raw :json + :json_polyfill since the discussion in Replace Jason with :json #666 leaned this way and it provides JSON.Encoder protocol which simplifies the migration
• The JSON.encode_to_iodata!/1 function is available in Elixir 1.18, keeping the iodata optimization for HTTP request bodies
• Util.safe_atom_map/1 is functionally equivalent to Jason's keys: :atoms — it recursively converts string keys to atoms with safety checks

Testing

• mix compile --warnings-as-errors passes
• mix test — 212 tests, 0 failures

Closes #666