feat: Enhance PostgreSQL installation with image pulling

- Pull the PostgreSQL image before installation to ensure the latest
  version is used. This improves reliability and reduces the chance of
  using outdated images.  Improves the robustness of the installation
  process.
- Added comprehensive unit tests for `PostgresInstallerConfig`,
  `PostgresInstallerError`, `install_postgres`, `create_database`,
  `execute_sql`, and `is_postgres_running` functions to ensure
  correctness and handle potential errors effectively.  Improves code
  quality and reduces the risk of regressions.

src/postgresclient/installer.rs

@@ -168,6 +168,20 @@ pub fn install_postgres(
     // Build the image name
     let image = format!("postgres:{}", config.version);
 
+    // Pull the PostgreSQL image to ensure we have the latest version
+    println!("Pulling PostgreSQL image: {}...", image);
+    let pull_result = Command::new("nerdctl")
+        .args(&["pull", &image])
+        .output()
+        .map_err(|e| PostgresInstallerError::IoError(e))?;
+
+    if !pull_result.status.success() {
+        return Err(PostgresInstallerError::NerdctlError(format!(
+            "Failed to pull PostgreSQL image: {}",
+            String::from_utf8_lossy(&pull_result.stderr)
+        )));
+    }
+
     // Create the container
     let mut container = Container::new(&config.container_name).map_err(|e| {
         PostgresInstallerError::NerdctlError(format!("Failed to create container: {}", e))