Function validate_prompt_content 

pub fn validate_prompt_content(content: &str) -> ValidationResult

Validates prompt content for common issues.

Checks for:

• Unclosed braces (e.g., {{var without closing }})
• Invalid variable names (non-alphanumeric characters)
• Reserved variable names (e.g., subcog_*, system_*, __*)
• Duplicate variable definitions

Returns

A validation result indicating whether the content is valid.