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/markdown.go

@@ -16,6 +16,7 @@ import (
 	"forgejo.org/modules/markup/common"
 	"forgejo.org/modules/markup/markdown/callout"
 	"forgejo.org/modules/markup/markdown/math"
+	markdownutil "forgejo.org/modules/markup/markdown/util"
 	"forgejo.org/modules/setting"
 	giteautil "forgejo.org/modules/util"
 
@@ -34,11 +35,6 @@ var (
 	specMarkdownOnce sync.Once
 )
 
-var (
-	renderContextKey = parser.NewContextKey()
-	renderConfigKey  = parser.NewContextKey()
-)
-
 type limitWriter struct {
 	w     io.Writer
 	sum   int64
@@ -64,7 +60,7 @@ func (l *limitWriter) Write(data []byte) (int, error) {
 // newParserContext creates a parser.Context with the render context set
 func newParserContext(ctx *markup.RenderContext) parser.Context {
 	pc := parser.NewContext(parser.WithIDs(newPrefixedIDs()))
-	pc.Set(renderContextKey, ctx)
+	pc.Set(markdownutil.RenderContextKey, ctx)
 	return pc
 }
 
@@ -192,7 +188,7 @@ func actualRender(ctx *markup.RenderContext, input io.Reader, output io.Writer)
 	}
 	rc.metaLength = metaLength
 
-	pc.Set(renderConfigKey, rc)
+	pc.Set(markdownutil.RenderConfigKey, rc)
 
 	if err := converter.Convert(buf, lw, parser.WithContext(pc)); err != nil {
 		log.Error("Unable to render: %v", err)