fix: consistent model path validation and error messages

[echobt]

Description

This PR addresses inconsistent model path validation behavior where invalid paths were sometimes accepted, leading to cryptic errors later during execution.

Changes

1. Updated embedding_model_path() and reranker_model_path() in src/config.rs to explicitly check for file existence.
2. Added descriptive error messages suggesting how to fix the issue (e.g., running vgrep models download).
3. Ensured that path resolution logic is centralized and consistent.

Testing

• Verified that providing a non-existent model path now returns a clear error immediately.
• Ran existing tests to ensure no regressions.

Closes #227