feat(trtllm): check existence of config files

When the required config files are not present, nlohmann/json throws
parsing error, which does not help much for identifying what was wrong.
Check the existence of these files early and return specific error
messages.

backends/trtllm/src/errors.rs

@@ -19,4 +19,8 @@ pub enum TensorRtLlmBackendError {
     WebServer(#[from] server::WebServerError),
     #[error("Tokio runtime failed to start: {0}")]
     Tokio(#[from] std::io::Error),
+    #[error("config.json doesn't exist in engine folder {0}")]
+    ConfigNotFound(PathBuf),
+    #[error("generation_config.json doesn't exist in engine folder {0}")]
+    GenerationConfigNotFound(PathBuf),
 }

backends/trtllm/src/looper.rs

@@ -3,7 +3,7 @@ use cxx::UniquePtr;
 use hashbrown::HashMap;
 use std::hint;
 use std::ops::Deref;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use tokenizers::Tokenizer;
 use tokio::sync::mpsc::{unbounded_channel, UnboundedReceiver, UnboundedSender};
 use tokio::sync::TryAcquireError;
@@ -283,6 +283,26 @@ fn ensure_paths_exist<P: AsRef<Path>, PP: AsRef<Path>>(
         return Err(err);
     }
 
+    let mut config_path = PathBuf::from(engine_folder);
+    config_path.push("config.json");
+
+    if !config_path.exists() {
+        let err = TensorRtLlmBackendError::ConfigNotFound(engine_folder.to_path_buf());
+
+        error!("Path validation failed: {}", err,);
+        return Err(err);
+    }
+
+    let mut generation_config_path = PathBuf::from(engine_folder);
+    generation_config_path.push("generation_config.json");
+
+    if !generation_config_path.exists() {
+        let err = TensorRtLlmBackendError::GenerationConfigNotFound(engine_folder.to_path_buf());
+
+        error!("Path validation failed: {}", err,);
+        return Err(err);
+    }
+
     // Ensure executor worker binary exists
     if !executor_worker_path.exists() {
         let err = TensorRtLlmBackendError::ExecutorWorkerNotFound(engine_folder.to_path_buf());