Add manual transaction API with commit and automatic rollback support

[TonyMasson]

Summary

• Add Transaction struct with explicit commit() method for manual transaction control
• Implement automatic rollback via Drop trait when transaction is not committed
• Add begin_transaction() method to Database for creating manual transactions
• Include comprehensive unit tests for both commit and rollback scenarios

Changes

• Database::begin_transaction() - creates a new manual transaction
• Transaction::commit() - explicitly commits the transaction
• Transaction Drop implementation - automatically rolls back if not committed
• Unit tests covering both commit and rollback behavior
• Updated existing tests to use new collection APIs

API Design

The new Transaction API complements the existing in_transaction() callback approach:

• Manual transactions: More flexible for complex logic and cross-function usage
• Callback transactions: Simpler for basic cases with automatic cleanup

Usage Example

// Manual transaction with explicit commit
let mut db = Database::open("mydb", None)?;
let transaction = db.begin_transaction()?;
// ... perform operations ...
transaction.commit()?;

// Automatic rollback via Drop
{
    let _transaction = db.begin_transaction()?;
    // ... perform operations ...
} // automatic rollback here

Test plan

• Unit tests for successful transaction commit
• Unit tests for automatic rollback on Drop
• All existing tests still pass
• Code formatting and linting checks pass

🤖 Generated with Claude Code

[Nao-ris]

Amazing :-)