Missing input validation in embedding providers

[RobertLD]

The Ollama, OpenAI, and local embedding providers accept arbitrary text input without validation:

• No check for empty strings
• No length limits before sending to external APIs
• No validation that API responses match expected schema (e.g., response.embeddings could be missing)

Suggested fix: Add input validation (non-empty, length limits) and runtime validation of API response shapes using zod or manual checks.