Add MongoDB transactions support

[anitnilay20]

Description

Implement MongoDB multi-document transactions with proper error handling and retry logic.

Requirements

• Multi-document transaction support
• Transaction retry logic
• Read/write concerns configuration
• Session management
• Causal consistency
• Write comprehensive tests

Acceptance Criteria

• Full transaction support for MongoDB 4.0+
• Automatic retry on transient errors
• Proper session handling
• Read/write concern configuration

Files to create

• beatrix_core/src/mongo/transaction.rs

Example Usage

db.transaction(|session| async move {
    let user = User::new("John".to_string());
    user.save_with_session(&db, &session).await?;
    
    let profile = Profile::new(user.id());
    profile.save_with_session(&db, &session).await?;
    
    Ok((user, profile))
}).await?;