Fix transaction leak when using Session in Scopes

[Copilot]

This PR fixes a transaction leak that occurs when using Session within Scopes. The issue manifests as database connections that are never properly committed or rolled back, leading to connection pool exhaustion.

Problem

When using the following pattern:

DB.Scopes(func(s *gorm.DB) *gorm.DB {
    return s.Session(&gorm.Session{
        DryRun: true,
    })
}).Where("name = ?", "jinzhu").Delete(&User{})

Transactions would leak because:

1. Session() sets clone=2 for non-NewDB configurations
2. BeginTransaction calls InstanceSet("gorm:started_transaction", true) which creates a new cloned instance and stores the transaction flag there
3. CommitOrRollbackTransaction calls InstanceGet("gorm:started_transaction") on the original instance, missing the transaction flag
4. Without the flag, transactions are never committed/rolled back, causing connection leaks

Root Cause

The issue stems from the difference between InstanceSet and InstanceGet behavior with cloned instances:

• InstanceSet calls getInstance() which can return a new cloned instance when clone > 0
• InstanceGet reads from the current instance directly
• This creates a mismatch where the flag is stored in one instance but retrieved from another

Solution

Modified the transaction callbacks to use direct Statement.Settings access instead of the InstanceSet/InstanceGet methods:

• BeginTransaction: db.Statement.Settings.Store("gorm:started_transaction", true)
• CommitOrRollbackTransaction: db.Statement.Settings.Load("gorm:started_transaction")

This ensures both callbacks operate on the same Statement instance, preventing the transaction leak.

Testing

Added comprehensive tests that:

• Reproduce the transaction leak issue using connection pool statistics
• Verify the fix resolves the leak
• Confirm existing transaction and scopes functionality remains intact

Fixes #7592.

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[horror-proton]

@jinzhu any other plan or idea for fixing #7592