chore(release): consolidate v3.12.3 release notes, changelog and test fixes

Merge previously unreleased v3.12.4 content into v3.12.3:
- CHANGELOG: combine [Unreleased] into [3.12.3], clear [Unreleased]
- Release notes (zh/en/ja): add Copilot proxy, macOS signing,
  Reasoning Effort, OpenCode SQLite, Codex 1M toggle, Disable
  Auto-Upgrade toggle, and contributor thanks
- Fix test mocks for skill backup/restore hooks
- Fix schema migration test missing providers table
- Fix TempHome to save/restore CC_SWITCH_TEST_HOME env var

src-tauri/src/database/tests.rs

@@ -296,6 +296,14 @@ fn schema_migration_v4_adds_pricing_model_columns() {
     let conn = Connection::open_in_memory().expect("open memory db");
     conn.execute_batch(
         r#"
+        CREATE TABLE providers (
+            id TEXT NOT NULL,
+            app_type TEXT NOT NULL,
+            name TEXT NOT NULL,
+            settings_config TEXT NOT NULL DEFAULT '{}',
+            meta TEXT NOT NULL DEFAULT '{}',
+            PRIMARY KEY (id, app_type)
+        );
         CREATE TABLE proxy_config (app_type TEXT PRIMARY KEY);
         CREATE TABLE proxy_request_logs (request_id TEXT PRIMARY KEY, model TEXT NOT NULL);
         CREATE TABLE mcp_servers (