tui: show rollback turn-context diffs

[charley-oai]

Summary

• extend rollback events with the surviving historical TurnContextItem so clients can compare rollback history against the current live session config
• show rollback context-diff info messages in the transcript and include the same diff block in the backtrack transcript overlay for turns captured in the current TUI session
• add the minimal constructor and tui_app_server compatibility updates required by the new protocol fields on top of current main

Testing

• cargo test -p codex-protocol
• cargo test -p codex-app-server-protocol
• cargo test -p codex-state
• cargo test -p codex-core
• cargo test -p codex-tui
• cargo test -p codex-tui-app-server
• just fix -p codex-protocol
• just fix -p codex-app-server-protocol
• just fix -p codex-state
• just fix -p codex-core
• just fix -p codex-tui
• just fix -p codex-tui-app-server
• just fmt

cargo test -p codex-core still hits the pre-existing tools::js_repl::tests::js_repl_imported_local_files_can_access_repl_globals tempdir permission failure while downloading dotslash.

[charley-oai]

Superseded by #14733 after restacking that branch onto #14822.