fix: valid XSD generation with model-driven namespaces

[suleman-uzair]

This PR fixes the invalid XSD produced by Model.to_xsd, which referenced the
xs: prefix without ever binding it, so no XSD processor could load the
output. It binds the XML Schema vocabulary to xmlns:xs and derives all
namespace handling from the models' XmlNamespace classes, so schemas spanning
multiple namespaces are generated correctly.

What this changes

• binds the XSD vocabulary to xmlns:xs and emits xs:-prefixed structure elements
• collects every namespace in the model tree from the XmlNamespace classes and declares xmlns:<prefix> for each
• sets targetNamespace from the root model and qualifies named-type references by their owning namespace
• emits deduplicated xs:import for foreign namespaces instead of folding their types into the root schema
• synthesizes a tns prefix when the target namespace declares none
• raises clear errors on prefix collisions and foreign namespaces without a usable prefix
• guards recursive models (A -> B -> A) against infinite recursion in validation and type generation
• adds specs covering the above, including schema-load checks

Type::Value type-namespace prefixing is left as a follow-up (documented in-code).

closes #717

[Copilot]

Pull request overview

Fixes invalid XSD output from to_xsd by explicitly binding the W3C XML Schema vocabulary to xmlns:xs and consistently emitting xs:-prefixed schema structure elements, while improving namespace qualification/import behavior for named types under target namespaces (regression coverage for #717).

Changes:

• Bind XSD vocabulary as xmlns:xs and prefix emitted schema structure elements (xs:schema, xs:element, xs:complexType, etc.).
• Qualify named-type QName references with the appropriate namespace prefix (target namespace or referenced model namespace), including collection cases, with cycle guards for recursive models.
• Add/adjust Nokogiri-load regression tests and update existing spec expectations to the new xs:-prefixed structure output.

Reviewed changes

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Show a summary per file

File	Description
lib/lutaml/xml/schema/xsd_schema.rb	Implements xs:-prefixed schema element emission, namespace declaration/collection, QName qualification, import logic, and recursion guards.
spec/lutaml/model/schema/xsd_schema_spec.rb	Adds Nokogiri schema-load regression coverage for #717 and additional namespace edge cases.
spec/lutaml/model/xsd_type_validation_spec.rb	Updates expectations to reflect xs:schema + xmlns:xs binding.
spec/lutaml/model/xsd_patterns_spec.rb	Updates pattern-based XSD string expectations for xs:-prefixed structure elements.
spec/lutaml/model/xsd_form_default_patterns_spec.rb	Updates expectations for xs:-prefixed structure elements in generated schemas.

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