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/app/plugin_hooks_test.go

@@ -185,7 +185,7 @@ func TestHookMessageWillBePosted(t *testing.T) {
 		require.Nil(t, err)
 
 		assert.Equal(t, "message", post.Message)
-		retrievedPost, errSingle := th.App.Srv().Store().Post().GetSingle(post.Id, false)
+		retrievedPost, errSingle := th.App.Srv().Store().Post().GetSingle(th.Context, post.Id, false)
 		require.NoError(t, errSingle)
 		assert.Equal(t, "message", retrievedPost.Message)
 	})
@@ -229,7 +229,7 @@ func TestHookMessageWillBePosted(t *testing.T) {
 		require.Nil(t, err)
 
 		assert.Equal(t, "message_fromplugin", post.Message)
-		retrievedPost, errSingle := th.App.Srv().Store().Post().GetSingle(post.Id, false)
+		retrievedPost, errSingle := th.App.Srv().Store().Post().GetSingle(th.Context, post.Id, false)
 		require.NoError(t, errSingle)
 		assert.Equal(t, "message_fromplugin", retrievedPost.Message)
 	})
@@ -1606,7 +1606,7 @@ func TestHookMessagesWillBeConsumed(t *testing.T) {
 		_, err := th.App.CreatePost(th.Context, newPost, th.BasicChannel, false, true)
 		require.Nil(t, err)
 
-		post, err := th.App.GetSinglePost(newPost.Id, true)
+		post, err := th.App.GetSinglePost(th.Context, newPost.Id, true)
 		require.Nil(t, err)
 		assert.Equal(t, "message", post.Message)
 	})
@@ -1629,7 +1629,7 @@ func TestHookMessagesWillBeConsumed(t *testing.T) {
 		_, err := th.App.CreatePost(th.Context, newPost, th.BasicChannel, false, true)
 		require.Nil(t, err)
 
-		post, err := th.App.GetSinglePost(newPost.Id, true)
+		post, err := th.App.GetSinglePost(th.Context, newPost.Id, true)
 		require.Nil(t, err)
 		assert.Equal(t, "mwbc_plugin:message", post.Message)
 	})