refactor: Add newtype for Topic

[tomverelst]

Problem

Topics are represented as plain String throughout the codebase. This allows invalid values (empty strings, invalid characters) to propagate.

Suggestion

Create a newtype with validation:

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Topic(String);

impl Topic {
    pub fn new(s: impl Into<String>) -> Result<Self, ValidationError> {
        let s = s.into();
        if s.is_empty() {
            return Err(ValidationError::EmptyTopic);
        }
        // Kafka topic name rules: alphanumeric, '.', '_', '-'
        if !s.chars().all(|c| c.is_alphanumeric() || c == '.' || c == '_' || c == '-') {
            return Err(ValidationError::InvalidTopicCharacters);
        }
        Ok(Topic(s))
    }
    
    pub fn as_str(&self) -> &str {
        &self.0
    }
}

Impact

• Catches invalid topic names at parse time
• Makes APIs self-documenting
• Prevents bugs from invalid string manipulation