fix: resolve ESBMC binary path from $PATH and print config file on load

Allow setting verifier.esbmc.path to just "esbmc" by resolving it via
shutil.which() before pydantic's FilePath validation. Also print which
config file is being loaded during startup.

Author: Yiannis128

esbmc_ai/config.py

@@ -8,6 +8,7 @@
 import logging
 import os
 from pathlib import Path
+import shutil
 
 from pydantic_settings import (
     BaseSettings,
@@ -292,6 +293,10 @@ class ESBMCConfig(BaseModel):
     def on_set_path(cls, value: FilePath | None) -> Path | None:
         if value is None:
             return None
+        # Try resolving the command from $PATH (e.g. "esbmc" without a full path).
+        which_path: str | None = shutil.which(str(value))
+        if which_path:
+            return Path(which_path)
         return Path(value).expanduser()
 
     params: list[str] = Field(
@@ -616,6 +621,7 @@ def settings_customise_sources(
         if config_file_path:
             config_file = Path(config_file_path).expanduser()
             if config_file.exists():
+                print(f"Loading config file: {config_file}")
                 sources.append(TomlConfigSettingsSource(settings_cls, config_file))
 
         # Add environment-based sources