Fix context usage (#33554)

Some old code use direct type-casting to get context, it causes
problems.

This PR fixes all legacy problems and use correct `ctx.Value` to get
low-level contexts.

Fix #33518

services/markup/renderhelper_codepreview.go

@@ -36,8 +36,8 @@ func renderRepoFileCodePreview(ctx context.Context, opts markup.RenderCodePrevie
 		return "", err
 	}
 
-	webCtx, ok := ctx.Value(gitea_context.WebContextKey).(*gitea_context.Context)
-	if !ok {
+	webCtx := gitea_context.GetWebContext(ctx)
+	if webCtx == nil {
 		return "", fmt.Errorf("context is not a web context")
 	}
 	doer := webCtx.Doer