feat(codex): add unified session history toggle for official providers

Codex buckets resume history by the model_provider id recorded in each
session: official runs (no key, built-in "openai") and cc-switch
third-party runs (shared "custom") are mutually invisible in the resume
picker. Add an opt-in setting that runs official providers under the
shared "custom" id so future official sessions land in the same history
bucket as third-party ones. Forward-only by design: existing sessions
are not migrated.

When enabled, official live config.toml gets model_provider = "custom"
plus a [model_providers.custom] entry that mirrors the built-in openai
provider (requires_openai_auth routes auth to the ChatGPT login in
auth.json, name "OpenAI" keeps is_openai() feature gates, explicit
supports_websockets/wire_api restore built-in defaults). auth.json is
untouched.

Key invariants:
- Injection lives only in the live config: switch-away backfill strips
  the exact injected shape, so stored provider configs stay clean and
  turning the toggle off fully reverts on the next write.
- Toggle changes apply immediately via a takeover-aware reapply: when
  the proxy owns the live config (backup/placeholder present), only the
  live backup is updated, mirroring the provider-switch path.
- The takeover backup path runs the same injection so a takeover
  release restores a config that still carries the unified routing.
- Injection refuses to activate a foreign [model_providers.custom]
  table (e.g. stale entry with a third-party base_url) to avoid routing
  ChatGPT OAuth traffic to an unknown backend.

The toggle lives under Settings → Codex App Enhancements; the
description warns that resuming old sessions across providers may fail
because encrypted_content reasoning only decrypts on the backend that
created it (upstream treats cross-provider resume as unsupported).

src-tauri/src/settings.rs

@@ -357,6 +357,12 @@ pub struct AppSettings {
     /// Opt-in: defaults to false so third-party switches cleanly overwrite auth.json.
     #[serde(default)]
     pub preserve_codex_official_auth_on_switch: bool,
+    /// Run official Codex providers under the shared "custom" model_provider id
+    /// so future official sessions share one resume-history bucket with
+    /// third-party providers. Forward-only: existing sessions are not migrated.
+    /// Opt-in: defaults to false.
+    #[serde(default)]
+    pub unify_codex_session_history: bool,
     /// User has confirmed the failover toggle first-run notice
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub failover_confirmed: Option<bool>,
@@ -475,6 +481,7 @@ impl Default for AppSettings {
             stream_check_confirmed: None,
             enable_failover_toggle: false,
             preserve_codex_official_auth_on_switch: false,
+            unify_codex_session_history: false,
             failover_confirmed: None,
             first_run_notice_confirmed: None,
             common_config_confirmed: None,
@@ -829,6 +836,16 @@ pub fn preserve_codex_official_auth_on_switch() -> bool {
         .preserve_codex_official_auth_on_switch
 }
 
+pub fn unify_codex_session_history() -> bool {
+    settings_store()
+        .read()
+        .unwrap_or_else(|e| {
+            log::warn!("设置锁已毒化，使用恢复值: {e}");
+            e.into_inner()
+        })
+        .unify_codex_session_history
+}
+
 // ===== 当前供应商管理函数 =====
 
 /// 获取指定应用类型的当前供应商 ID（从本地 settings 读取）