CVE-2026-61741: http4s-scala-xml can expose files or network via XML

http4s-scala-xml provides `EntityDecoder[F, scala.xml.Elem]` instances that parse XML message bodies. These decoders used a `javax.xml.parsers.SAXParserFactory` obtained from `SAXParserFactory.newInstance` without any security configuration. With the JDK's default settings, the parser resolves DOCTYPE declarations, external general and parameter entities, and external DTDs.

An application that uses these decoders to parse untrusted XML is vulnerable to XML External Entity (XXE) attacks. An attacker can craft a request that:

- discloses local files readable by the service process,
- performs server-side request forgery (SSRF) against internal network resources, and/or
- causes denial of service through entity expansion.

### Impact

Any service that derives an XML `EntityDecoder` from this library and parses attacker-controlled input is affected.

### Patches

The default SAXParserFactory is now hardened:
- secure processing is enabled
- `DOCTYPE` declarations are disallowed
- external general/parameter entities and external DTD loading are disabled
- DTD URIs are not resolved
- XInclude is disabled.

These defaults match `scala.xml.XMLLoader`'s defaults since 2.0.0.

### Workarounds

Before upgrading, override `ElemInstances#saxFactory` with a hardened factory.

```scala
val secured = new ElemInstances {
override protected val saxFactory = {
val f = javax.xml.parsers.SAXParserFactory.newInstance
f.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true)
// etc.
f
}
}
import secured._
```