Problem Statement

The configuration loader uses os.ExpandEnv, which silently replaces missing environment variables with empty strings. This leads to cryptic errors from providers (e.g., "API key is empty") rather than a clear diagnostic message about which environment variable is missing.

Proposed Solution

• Add a Validate() error method to the Config struct in internal/core/config/config.go.
• Call validation after loading the configuration.
• Check for required fields like Qdrant.URL, Qdrant.APIKey, and Embedding.APIKey.
• Provide clear error messages indicating which environment variable or config field is missing.