Generate XSI namespace constants for nillable type support

[Copilot]

Generated code referenced undefined PREFIX_XSI and NS_XSI constants when combining Global namespace serialization with nillable types, causing compilation errors.

Changes

Constant Generation

• Modified namespace_const.rs and prefix_const.rs to emit NS_XSI and PREFIX_XSI when NILLABLE_TYPE_SUPPORT is enabled
• Constants now generate even for schemas without explicit namespace definitions

Path Resolution

• Updated serialize.rs to use PathData and resolve_type_for_serialize_module() for XSI constants
• Ensures correct module path depth (super::, super::super::, etc.) matching other namespace constants

Test Coverage

• Added nillable_global_namespace test suite validating the fix
• Updated expectations for existing tests using nillable types

Example

Before this fix, generated code would fail to compile:

// Generated code (broken)
helper.write_xmlns(&mut bytes, Some(&super::PREFIX_XSI), &super::NS_XSI);
// error: cannot find value `PREFIX_XSI` in module `super`

Now generates:

// At module root
pub const PREFIX_XSI: NamespacePrefix = NamespacePrefix::new_const(b"xsi");
pub const NS_XSI: Namespace = Namespace::new_const(b"http://www.w3.org/2001/XMLSchema-instance");

// In serialization code (nested module)
helper.write_xmlns(&mut bytes, Some(&super::super::PREFIX_XSI), &super::super::NS_XSI);

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add tests to verify XSI namespace is working correctly</issue_title>
<issue_description>Two issues needs to be verified:

1. When explicitly adding the XSI schema to the parser, a error is raised:

    config.parser.schemas = vec![
        // Build started to fail after i added any of them.
        Schema::Url(url::Url::parse(http://www.w3.org/2001/XMLSchema-instance).unwrap()),
        Schema::File("xsd/xsi-instance.xsd".into()),
    ];
    config.parser.resolver = vec![Resolver::Web, Resolver::File];

2. The XSI_PREFIX constant is not defined and cause compiler errors in the generated code:

RenderStep::QuickXmlSerialize {
    namespaces: NamespaceSerialization::Global,
    default_namespace: None,
}

helper.write_xmlns(
    &mut bytes,
    Some(&super::PREFIX_XSI),
    &super::NS_XSI,
);
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Add tests to verify XSI namespace is working correctly #216

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[Bergmann89]

Will be addressed in #233. Closed.