Why

Fixes #20145.

config/value/write treats a JSON null value as a request to clear the config key. Clearing a key that is already absent should be idempotent, but clearing a nested key such as features.personality from an empty config.toml returned configPathNotFound because clear_path treated the missing features parent table as an error.

That makes app-server reset flows brittle because clients have to read first and avoid sending a clear request unless the parent path already exists.

What Changed

• Updated app-server config clearing so missing intermediate tables, or non-table parents, are treated as an unchanged no-op.
• Removed the now-unreachable MergeError::PathNotFound path from config write merging.
• Added a regression test covering features.personality = null against an empty user config.

Verification

• cargo test -p codex-app-server clear_missing_nested_config_is_noop
• cargo test -p codex-app-server was run; the config manager unit suite passed, but one unrelated integration test failed because turn_start_emits_thread_scoped_warning_notification_for_trimmed_skills expected 7 trimmed skills and observed 8.
• just fix -p codex-app-server