fix(mapper): handle struct payloads in stringify_keys and simplify rate-limit fixture/test

[icostan]

Summary

• Fix mapper crash when codex_sdk emits struct payloads (notably Codex.Protocol.RateLimit.Snapshot)
  under app-server transport.
• Update stringify_keys/1 to handle structs safely before recursive key normalization.
• Update fixtures/tests to align with latest codex_sdk rate-limit payload shape.

Changes

• Add struct handling path in mapper:
  • convert structs via Map.from_struct/1
  • continue recursive key stringification
• Update rate_limits_updated fixture to return a minimal RateLimit.Snapshot.
• Update mapper tests to assert snapshot-based payload mapping through fixtures.
• Keep dependency updates aligned with latest codex_sdk (mix.exs + mix.lock).

Why

mix codex.smoke ... --transport app_server could fail with:
Protocol.UndefinedError for Codex.Protocol.RateLimit.Snapshot in mapper key normalization.

Validation

• mix test
• mix quality
• Smoke run verified in terminal:
  • mix codex.smoke "Say hello" --transport app_server
  • completed successfully with normalized events.