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/gtprof/trace_builtin.go

@@ -63,7 +63,7 @@ func (t *traceBuiltinSpan) toString(out *strings.Builder, indent int) {
 	}
 	out.WriteString("\n")
 	for _, c := range t.ts.children {
-		span := c.internalSpans[t.internalSpanIdx].(*traceBuiltinSpan)
+		span := c.internalSpans[t.internalSpanIdx].(*traceBuiltinSpan) //nolint:forcetypeassert // this tracer only stores its own spans
 		span.toString(out, indent+2)
 	}
 }