chore: enable forcetypeassert linter, fix issues (#38804)

Enable [`forcetypeassert`](https://github.com/gostaticanalysis/forcetypeassert)
linter to prevent unchecked type assertions. ~650 issues fixed, most
fixes were clean, some use `setting.PanicInDevOrTesting`.

The only behaviour changes are where code would previously send a 500 error
or panic, a 4xx error is now emitted.

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/markdown/math/block_parser.go

@@ -86,7 +86,7 @@ func (b *blockParser) Open(parent ast.Node, reader text.Reader, pc parser.Contex
 
 // Continue parses the current line and returns a result of parsing.
 func (b *blockParser) Continue(node ast.Node, reader text.Reader, pc parser.Context) parser.State {
-	block := node.(*Block)
+	block := node.(*Block) //nolint:forcetypeassert // this parser only ever opens *Block nodes
 	if block.Closed {
 		return parser.Close
 	}