app-server: remove experimental persist_extended_history bool flag

[owenlin0]

Summary

Remove the dead experimental persistExtendedHistory app-server flag and collapse rollout persistence to the single policy app-server already used.

What Changed

• Removed persistExtendedHistory from v2 thread start/resume/fork params and deleted its deprecation notice path.
• Removed the persistence-mode enums and plumbing through core, rollout, and thread-store.
• Made rollout filtering mode-free, keeping the existing limited persisted-history behavior.

Test Plan

• just write-app-server-schema
• cargo nextest run --no-fail-fast -p codex-app-server-protocol schema_fixtures
• cargo nextest run --no-fail-fast -p codex-app-server thread_shell_command_history_responses_exclude_persisted_command_executions
• cargo nextest run --no-fail-fast -p codex-rollout -p codex-thread-store
• final rg for removed flag/type names