docs: document Codex Chat provider support in changelog and manual

Backfill the empty [Unreleased] CHANGELOG section with the Codex Chat
Completions feature (Chat-to-Responses bridge, 23 third-party presets,
model mapping table, Stream Check routing, error-envelope conversion,
"custom" history bucket) plus the community fixes landed since v3.15.0.

Update the zh/en/ja user manual: split the Codex preset tables by
upstream protocol (native Responses vs Chat Completions), add a "Codex
Local Routing and Model Mapping" section covering the Needs Local
Routing toggle and the model catalog, and note Chat-format probing in
the Stream Check guide. Manual wording matches the live UI strings.