Validate portfolio file and create model portfolio or blend if valid

Parse model data, validate it, and create a model portfolio or blend if there are no validation errors.

Supported Identifiers:

• Ticker symbols (e.g., 'AAPL', 'MSFT') for securities
• Model portfolio IDs (UUIDs) for existing model portfolios

Types:

• model-portfolio: Creates a model portfolio with ticker-based securities only
• blend: Creates a blend that can contain both tickers and model portfolio IDs

Date Validation:

• For model portfolio IDs, validates that the model portfolio existed on the specified date
• For tickers, validates that the ticker was valid on the specified date