Fix Entity.SaveAsync method session queue issue with sharding

[Copilot]

The SaveAsync method in Entity<TEntity> had a critical bug when using database sharding (分表分库). The method was obtaining a reference to the session queue before creating the shard, but creating the shard changes the session context, making the queue reference stale.

Problem

// Before fix - problematic code:
var queue = Meta.Session.Queue;           // Gets queue from current session
queue.InsertOnly = Meta.Table.DataTable.InsertOnly;

if (Meta.InShard) return queue.Add(this, msDelay);

// This changes Meta.Session to point to a different session!
using var split = Meta.CreateShard((this as TEntity)!);

return queue.Add(this, msDelay);          // Still uses old queue!

When Meta.CreateShard() is called, it modifies Meta.ConnName and Meta.TableName, which causes the SessionWrap.Reset() method to nullify the _Session field. Subsequent calls to Meta.Session return a new session instance for the sharded table, but the queue variable still references the queue from the original session.

Solution

The fix restructures the queue acquisition to ensure the correct session queue is used:

// After fix - correct code:
if (Meta.InShard)
{
    // Already in shard context, use current queue
    var queue = Meta.Session.Queue;
    queue.InsertOnly = Meta.Table.DataTable.InsertOnly;
    return queue.Add(this, msDelay);
}

// Create shard first, then get the correct queue
using var split = Meta.CreateShard((this as TEntity)!);

// Now get queue from the updated session
var queue2 = Meta.Session.Queue;
queue2.InsertOnly = Meta.Table.DataTable.InsertOnly;
return queue2.Add(this, msDelay);

This ensures that entities are properly queued to the correct session when using sharding functionality, preventing data from being saved to the wrong database or table.

Fixes #69.

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