refactor(backend): phase 3 - unify error handling and fix backup timestamp bug

Key improvements:
- Extract switch_provider_internal() returning AppError for better testability
- Fix backup mtime inheritance: use read+write instead of fs::copy to ensure latest backup survives cleanup
- Add 15+ integration tests covering provider commands, atomic writes, and rollback scenarios
- Expose write_codex_live_atomic, AppState, and test hooks in public API
- Extract tests/support.rs with isolated HOME and mutex utilities

Test coverage:
- Provider switching with live config backfill and MCP sync
- Codex atomic write success and failure rollback
- Backup retention policy with proper mtime ordering
- Negative cases: missing auth field, invalid provider ID

src-tauri/tests/provider_commands.rs

@@ -0,0 +1,159 @@
+use serde_json::json;
+
+use cc_switch_lib::{
+    get_codex_auth_path, get_codex_config_path, read_json_file, switch_provider_test_hook,
+    write_codex_live_atomic, AppState, AppType, MultiAppConfig, Provider,
+};
+
+#[path = "support.rs"]
+mod support;
+use support::{ensure_test_home, reset_test_fs, test_mutex};
+
+#[test]
+fn switch_provider_updates_codex_live_and_state() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let _home = ensure_test_home();
+
+    let legacy_auth = json!({"OPENAI_API_KEY": "legacy-key"});
+    let legacy_config = r#"[mcp_servers.legacy]
+type = "stdio"
+command = "echo"
+"#;
+    write_codex_live_atomic(&legacy_auth, Some(legacy_config))
+        .expect("seed existing codex live config");
+
+    let mut config = MultiAppConfig::default();
+    {
+        let manager = config
+            .get_manager_mut(&AppType::Codex)
+            .expect("codex manager");
+        manager.current = "old-provider".to_string();
+        manager.providers.insert(
+            "old-provider".to_string(),
+            Provider::with_id(
+                "old-provider".to_string(),
+                "Legacy".to_string(),
+                json!({
+                    "auth": {"OPENAI_API_KEY": "stale"},
+                    "config": "stale-config"
+                }),
+                None,
+            ),
+        );
+        manager.providers.insert(
+            "new-provider".to_string(),
+            Provider::with_id(
+                "new-provider".to_string(),
+                "Latest".to_string(),
+                json!({
+                    "auth": {"OPENAI_API_KEY": "fresh-key"},
+                    "config": r#"[mcp_servers.latest]
+type = "stdio"
+command = "say"
+"#
+                }),
+                None,
+            ),
+        );
+    }
+
+    config.mcp.codex.servers.insert(
+        "echo-server".into(),
+        json!({
+            "id": "echo-server",
+            "enabled": true,
+            "server": {
+                "type": "stdio",
+                "command": "echo"
+            }
+        }),
+    );
+
+    let app_state = AppState {
+        config: std::sync::Mutex::new(config),
+    };
+
+    switch_provider_test_hook(&app_state, AppType::Codex, "new-provider")
+        .expect("switch provider should succeed");
+
+    let auth_value: serde_json::Value =
+        read_json_file(&get_codex_auth_path()).expect("read auth.json");
+    assert_eq!(
+        auth_value
+            .get("OPENAI_API_KEY")
+            .and_then(|v| v.as_str())
+            .unwrap_or(""),
+        "fresh-key",
+        "live auth.json should reflect new provider"
+    );
+
+    let config_text =
+        std::fs::read_to_string(get_codex_config_path()).expect("read config.toml");
+    assert!(
+        config_text.contains("mcp_servers.echo-server"),
+        "config.toml should contain synced MCP servers"
+    );
+
+    let locked = app_state
+        .config
+        .lock()
+        .expect("lock config after switch");
+    let manager = locked
+        .get_manager(&AppType::Codex)
+        .expect("codex manager after switch");
+    assert_eq!(manager.current, "new-provider", "current provider updated");
+
+    let new_provider = manager
+        .providers
+        .get("new-provider")
+        .expect("new provider exists");
+    let new_config_text = new_provider
+        .settings_config
+        .get("config")
+        .and_then(|v| v.as_str())
+        .unwrap_or_default();
+    assert_eq!(
+        new_config_text, config_text,
+        "provider config snapshot should match live file"
+    );
+
+    let legacy = manager
+        .providers
+        .get("old-provider")
+        .expect("legacy provider still exists");
+    let legacy_auth_value = legacy
+        .settings_config
+        .get("auth")
+        .and_then(|v| v.get("OPENAI_API_KEY"))
+        .and_then(|v| v.as_str())
+        .unwrap_or("");
+    assert_eq!(
+        legacy_auth_value, "legacy-key",
+        "previous provider should be backfilled with live auth"
+    );
+}
+
+#[test]
+fn switch_provider_missing_provider_returns_error() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+
+    let mut config = MultiAppConfig::default();
+    config
+        .get_manager_mut(&AppType::Claude)
+        .expect("claude manager")
+        .current = "does-not-exist".to_string();
+
+    let app_state = AppState {
+        config: std::sync::Mutex::new(config),
+    };
+
+    let err = switch_provider_test_hook(&app_state, AppType::Claude, "missing-provider")
+        .expect_err("switching to a missing provider should fail");
+
+    assert!(
+        err.to_string().contains("供应商不存在"),
+        "error message should mention missing provider"
+    );
+}