feat: implement transaction handling for upstream and target operations

- Added transaction support in `add_upstream_target`, `remove_upstream`, `remove_upstream_target`, `update_upstream`, and `update_upstream_target` functions to ensure atomicity of operations.
- Updated the `NginxService` to include methods for validating and applying configurations using the agent service.
- Enhanced error handling in agent service interactions, returning appropriate internal server errors when agent communication fails.
- Introduced mock agent service for testing, allowing for simulation of agent interactions without actual network calls.
- Refactored tests to cover scenarios where agent operations fail, ensuring that internal server errors are returned as expected.

apps/api/src/services/nginx/builder.rs

@@ -6,17 +6,12 @@ pub trait NginxConfigProvider {
     fn to_nginx_config(&self, indent: Option<usize>) -> String;
 }
 
+#[derive(Default)]
 pub struct NginxConfigBuilder {
     upstreams: Vec<UpstreamInfo>,
 }
 
 impl NginxConfigBuilder {
-    pub fn new() -> Self {
-        Self {
-            upstreams: Vec::new(),
-        }
-    }
-
     pub fn add_upstream(&mut self, upstream: UpstreamInfo) {
         self.upstreams.push(upstream);
     }