COMMONSXML-7: SchemaFactory: capability-driven hardening - #13

Replace the per-implementation class-name dispatch for Schema with a single
wrapper-based recipe, the same for every implementation. The hardening lives
entirely in the existing HardeningSchemaFactory / HardeningSchema /
HardeningValidator wrappers, so there is no per-implementation branching, no
FEATURE_SECURE_PROCESSING and no limit configuration on the factory itself:

- HardeningSchemaFactory installs a deny-all LSResourceResolver on the factory
  (blocking xs:import/include/redefine at compile time) and rewrites every
  newSchema(Source[]) through an XmlFactories-hardened reader.
- HardeningSchema wraps every Validator/ValidatorHandler the inner Schema
  produces and re-installs the deny-all resolver on each (blocking
  xsi:schemaLocation at validation time), since neither the JDK nor Xerces
  reliably propagates it through Schema.
- HardeningValidator rewrites the Source on every validate(Source) call.

The hardened reader from XmlFactories.harden(Source) already carries
FEATURE_SECURE_PROCESSING and the processing limits, so a DOCTYPE, external
entity or Billion Laughs payload in the schema or instance document is bounded
there rather than on this factory.

The JAXP 1.5 ACCESS_EXTERNAL_* properties are deliberately not set: the deny-all
resolver already blocks the same fetches on every implementation, and the JDK 8
SchemaFactory has a bug whereby those properties keep blocking even when a
caller's own resolver would grant the access, so leaving them unset lets a
caller re-enable specific lookups by swapping the resolver. The JDK block now
surfaces as the resolver's SecurityException rather than a SAXException, which
the attack-test assertions already accept.

StockJdkProvider.configure(SchemaFactory), XercesProvider.configure(SchemaFactory)
with its per-product hardeners, XmlFactories.dispatch(SchemaFactory) and
Limits.applyToJdkSchema are removed; XmlFactories.newSchemaFactory() wraps in
HardeningSchemaFactory directly. An implementation is no longer rejected for
being unrecognized.

Assisted-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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