Core: XmlPullParser processes external DTD entities (XXE) — disable FEATURE_PROCESS_DOCDECL in Xml#createParser()

Start at Xml#createParser() in the google-http-client-xml module and inspect how the new XmlPullParser is configured. Run or add the named regression test, testCreateParser_disablesDocDecl, using the issue's DTD entity example. Done means document declarations are disabled or safely rejected, and the regression test confirms the entity is not resolved.

Environment details

1. Core (XML) — Xml#createParser() in google-http-client-xml
2. OS type and version: Any (JVM-level issue, not OS-specific)
3. Java version: Any (Java 8+)
4. google-http-client-xml version: 2.1.1 and earlier

Problem Statement

Xml#createParser() returns a raw XmlPullParser without disabling
XmlPullParser.FEATURE_PROCESS_DOCDECL. If an attacker can influence the XML content
parsed by the library (e.g. via a malicious server response or injected content), they can
embed a <!DOCTYPE> declaration with external entity references (),
causing the parser to resolve arbitrary external entities.

This is a classic XML External Entity (XXE) vulnerability.

Steps to reproduce

1. Call Xml.createParser() and feed it XML containing an inline DTD with entity
   declarations ( ]>).
2. Parse the document with Xml.parseElement(...).
3. Observe that the entity &xxe; is resolved and its value appears in the parsed output
   instead of being rejected or left unexpanded.

Code example

XmlPullParser parser = Xml.createParser();
String xmlWithDtd = "\n"
+ "\n"
+ "]>\n"
+ "&xxe;";
parser.setInput(new StringReader(xmlWithDtd));

SimpleTypeString result = new SimpleTypeString();
Xml.parseElement(parser, result, new XmlNamespaceDictionary().set("", ""), null);
// result.value == "injected" <-- DTD entity resolved, XXE confirmed

Security Impact

• Allows entity injection / content spoofing from malicious XML input.
• In environments parsing untrusted XML (e.g. API responses from attacker-controlled
  servers), this could lead to information disclosure or server-side request forgery (SSRF)
  if external URI entities are supported by the underlying parser implementation.

Proposed Fix

Set FEATURE_PROCESS_DOCDECL to false immediately after creating the parser, with a
graceful fallback for parsers that do not support the feature flag:

public static XmlPullParser createParser() throws XmlPullParserException {
  XmlPullParser parser = getParserFactory().newPullParser();
  try {
    parser.setFeature(XmlPullParser.FEATURE_PROCESS_DOCDECL, false);
  } catch (XmlPullParserException e) {
    // Ignore if the feature is not supported by this parser implementation
  }
  return parser;
}

A regression test (testCreateParser_disablesDocDecl) is included in the accompanying PR
verifying that DTD entity content is not resolved (or that the parser throws, which is also
an acceptable safe outcome).