Add structured validation for generated XAUUSD research reports

[ziyuwowo]

Currently, GoldFXGraph generates XAUUSD research reports through the LangGraph workflow, but the final report quality validation is still limited.

It would be useful to add a structured validation layer before the final report is persisted or returned to the user.

The validation could check:

• Whether all required report sections are present
• Whether forecast direction is valid
• Whether entry price, stop loss, and take profit are logically consistent
• Whether the confidence score is within a valid range
• Whether the report contains unsupported or contradictory statements
• Whether the generated output matches the expected Pydantic schema

Expected behavior:

If validation passes, the report can be persisted normally.

If validation fails, the workflow should either:

• Retry the report generation step, or
• Return a structured validation error with clear reasons
• This would improve the reliability and trustworthiness of the research workflow.