fix: enable multi-line math equations in wiki (#8424)

- When math equation support was added into Gitea it allowed for math equations to be typed over multiple lines via `\[ ... \]` and `$$ ... $$`. Specifically the former delimiters caused problems with legitimate markdown input to be seen as a math equation and therefore was disabled in e1a82a15d3.
- Enable this multi-line parsing for wiki as it's less likely to cause issues in the context of the wiki.
- It is hard to fix this issue in a proper way without investing a good amount of time in Goldmark as explained in https://codeberg.org/forgejo/forgejo/issues/6902#issuecomment-2845317
- Added unit test.
- Resolves forgejo/forgejo#6902

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8424
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/markup/markdown/goldmark.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"forgejo.org/modules/markup"
+	markdownutil "forgejo.org/modules/markup/markdown/util"
 	"forgejo.org/modules/setting"
 
 	"github.com/yuin/goldmark/ast"
@@ -35,8 +36,8 @@ func (g *ASTTransformer) applyElementDir(n ast.Node) {
 func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc parser.Context) {
 	firstChild := node.FirstChild()
 	tocMode := ""
-	ctx := pc.Get(renderContextKey).(*markup.RenderContext)
-	rc := pc.Get(renderConfigKey).(*RenderConfig)
+	ctx := pc.Get(markdownutil.RenderContextKey).(*markup.RenderContext)
+	rc := pc.Get(markdownutil.RenderConfigKey).(*RenderConfig)
 
 	tocList := make([]markup.Header, 0, 20)
 	if rc.yamlNode != nil {