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.go

@@ -21,8 +21,8 @@ func FormalRenderHelperFuncs() *markup.RenderHelperFuncs {
 				return false
 			}
 
-			giteaCtx, ok := ctx.(*gitea_context.Context)
-			if !ok {
+			giteaCtx := gitea_context.GetWebContext(ctx)
+			if giteaCtx == nil {
 				// when using general context, use user's visibility to check
 				return mentionedUser.Visibility.IsPublic()
 			}

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

services/markup/renderhelper_issueicontitle.go

@@ -18,8 +18,8 @@ import (
 )
 
 func renderRepoIssueIconTitle(ctx context.Context, opts markup.RenderIssueIconTitleOptions) (_ template.HTML, err error) {
-	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")
 	}