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

**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 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).

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.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.