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/math/block_parser.go

@@ -6,6 +6,9 @@ package math
 import (
 	"bytes"
 
+	"forgejo.org/modules/markup"
+	markdownutil "forgejo.org/modules/markup/markdown/util"
+
 	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/parser"
 	"github.com/yuin/goldmark/text"
@@ -61,6 +64,13 @@ func (b *blockParser) Open(parent ast.Node, reader text.Reader, pc parser.Contex
 		return node, parser.Close | parser.NoChildren
 	}
 
+	ctx := pc.Get(markdownutil.RenderContextKey).(*markup.RenderContext)
+	if ctx.IsWiki {
+		reader.Advance(segment.Len() - 1)
+		segment.Start += 2
+		node.Lines().Append(segment)
+		return node, parser.NoChildren
+	}
 	return nil, parser.NoChildren
 }