Revamp XML configuration testing and documentation

[ppkarwasz]

This PR reworks how the XML configuration format is tested and documented so that each behavior of the DOM-to-Node conversion in XmlConfiguration is covered by its own focused test. Doing so surfaced a small attribute-handling bug, fixed here.

Testing

• constructHierarchy, getType and processAttributes are now static and take the PluginManager and strict flag as parameters, so the DOM-to-Node conversion can be unit-tested with a mocked PluginManager instead of spinning up a full LoggerContext.
• The StrictXmlConfigTest integration test (which only asserted event counts and merely happened to exercise strict-mode parsing) is replaced by XmlConfigurationConstructHierarchyTest, covering individually: plugin resolution, attribute folding, text-as-value, strict-mode type resolution, and XInclude whole-file / positional element() / by-name (ID) fragment resolution.
• The unused Status/ErrorType bookkeeping is dropped in favor of logging the error directly.

Bug fix

• The XInclude fixup-language feature adds an xml:lang attribute to included elements that was never stripped (only xml:base was) and was not covered by any test, so it leaked through as a Log4j configuration attribute. processAttributes now matches the reserved XML namespace instead of the literal attribute name, stripping xml:base, xml:lang, and any other reserved xml: attribute.
• XmlConfiguration no longer explicitly enables the fixup-base-uris/fixup-language features: they default to true and only
  add attributes Log4j strips (they are not required for nested or relative includes to resolve).

Documentation

• The XML format section of configuration.adoc is moved into a reusable AsciiDoc partial (partials/manual/configuration-xml-format.adoc).
• The XInclude docs gain a section about including fragments with positional and by-name XPointer examples, plus a note that the JDK does not recognize xml:id and does not implement the xpointer() scheme.

There is no behavior change for valid configurations; the only runtime difference is that xml:lang no longer leaks into the configuration.

Checklist

• Based on 2.x
• ./mvnw verify succeeds (only the affected modules were tested locally)
• Changelog entry in src/changelog/.2.x.x
• Tests provided