feat: add agent module with Nginx service commands and routes

- Introduced a new agent module with commands for managing Nginx configurations.
- Implemented `NginxService` for handling reload, validation, and configuration writing.
- Added routes for status, validation, and configuration writing using Axum.
- Created necessary command files: `reload.rs`, `run.rs`, `validate.rs`, `write_config.rs`.
- Updated `Cargo.toml` and `Cargo.lock` to include new dependencies.
- Added `.gitignore` for the agent module.
- Updated `justfile` to include OpenAPI generation for the agent.

apps/agent/src/commands/validate.rs

@@ -0,0 +1,47 @@
+use crate::commands::{run::run_cmd, write_config::INTERNAL_CONFIG_FOLDER_NAME};
+use std::path::PathBuf;
+
+pub struct ValidateCommand {
+    nginx_config_dir: PathBuf,
+}
+
+impl ValidateCommand {
+    pub fn new(nginx_config_dir: PathBuf) -> Self {
+        Self { nginx_config_dir }
+    }
+
+    pub fn nginx_config_dir(&self) -> PathBuf {
+        self.nginx_config_dir.clone()
+    }
+
+    pub async fn validate_all(
+        &self,
+    ) -> Result<(i32, String), Box<dyn std::error::Error + Send + Sync>> {
+        run_cmd("nginx", &["-t"], 10).await
+    }
+
+    pub async fn validate(
+        &self,
+        config_name: &str,
+        timestamp: u64,
+    ) -> Result<(i32, String), Box<dyn std::error::Error + Send + Sync>> {
+        let filename = crate::commands::run::to_file_name(config_name, timestamp)?;
+        // fragments are written into the YANPM subdirectory
+        let full_path = self
+            .nginx_config_dir
+            .join(INTERNAL_CONFIG_FOLDER_NAME)
+            .join(&filename);
+
+        // ensure the fragment file exists
+        if tokio::fs::metadata(&full_path).await.is_err() {
+            return Err(format!("Config file not found: {}", full_path.display()).into());
+        }
+
+        run_cmd(
+            "nginx",
+            &["-t", "-c", full_path.to_str().ok_or("invalid config path")?],
+            10,
+        )
+        .await
+    }
+}