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/services/proxy.rs

@@ -2033,6 +2033,14 @@ impl ProxyService {
                 )?;
                 Self::preserve_codex_oauth_auth_in_backup(&mut effective_settings, existing_value)?;
             }
+
+            // 统一会话开关：备份是接管释放时恢复 live 的来源，官方配置的
+            // 共享 custom 路由注入必须落在备份里，否则恢复后开关失效。
+            crate::codex_config::apply_codex_unified_session_bucket_to_settings(
+                provider.category.as_deref(),
+                &mut effective_settings,
+            )
+            .map_err(|e| format!("注入统一会话路由失败: {e}"))?;
         }
 
         let backup_json = match app_type_enum {