refactor: 移除 Python upstream 依赖，清理全部 legacy/Python 兼容层

- 移除 upstream_base_url 参数及 AETHER_GATEWAY_UPSTREAM 环境变量，gateway 不再需要指向 Python 宿主
- 删除所有 LEGACY_*/PYTHON_* 常量、路由组、header 定义及 sunset/phaseout 机制
- 将 legacy_gateway_bridge 重命名为 internal_gateway，executor 相关命名统一为 execution_runtime
- dev.sh 新增 Postgres/Redis 预检查，移除 upstream 相关启动参数和提示
- 新增 ai_public 路由处理器
- 全量适配 handler、test、state、control 等模块的命名和接口变更

apps/aether-gateway/src/usage/reporting.rs

@@ -611,7 +611,7 @@ mod tests {
     }
 
     fn build_test_state(repository: Arc<InMemoryRequestCandidateRepository>) -> AppState {
-        AppState::new("http://upstream.example")
+        AppState::new()
             .expect("gateway state should build")
             .with_data_state_for_tests(
                 GatewayDataState::with_request_candidate_and_usage_repository_for_tests(
@@ -625,7 +625,7 @@ mod tests {
         video_repository: Arc<InMemoryVideoTaskRepository>,
         request_candidate_repository: Arc<InMemoryRequestCandidateRepository>,
     ) -> AppState {
-        AppState::new("http://upstream.example")
+        AppState::new()
             .expect("gateway state should build")
             .with_data_state_for_tests(
                 GatewayDataState::with_video_task_and_request_candidate_repository_for_tests(
@@ -639,8 +639,9 @@ mod tests {
         request_candidate_repository: Arc<InMemoryRequestCandidateRepository>,
         gemini_file_mapping_repository: Arc<InMemoryGeminiFileMappingRepository>,
     ) -> AppState {
-        AppState::new("http://upstream.example").expect("gateway state should build")
-        .with_data_state_for_tests(
+        AppState::new()
+            .expect("gateway state should build")
+            .with_data_state_for_tests(
             GatewayDataState::with_request_candidate_and_gemini_file_mapping_repository_for_tests(
                 request_candidate_repository,
                 gemini_file_mapping_repository,