MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/store/store.go

@@ -353,7 +353,7 @@ type PostStore interface {
 	Save(rctx request.CTX, post *model.Post) (*model.Post, error)
 	Update(rctx request.CTX, newPost *model.Post, oldPost *model.Post) (*model.Post, error)
 	Get(ctx context.Context, id string, opts model.GetPostsOptions, userID string, sanitizeOptions map[string]bool) (*model.PostList, error)
-	GetSingle(id string, inclDeleted bool) (*model.Post, error)
+	GetSingle(rctx request.CTX, id string, inclDeleted bool) (*model.Post, error)
 	Delete(rctx request.CTX, postID string, timestamp int64, deleteByID string) error
 	PermanentDeleteByUser(rctx request.CTX, userID string) error
 	PermanentDeleteByChannel(rctx request.CTX, channelID string) error