Fix #11715: preserve 4.1.0 namespace/schema in help:effective-pom

[arturobernalg]

When generating the effective POM for a modelVersion 4.1.0 project, preserve the root namespace and schemaLocation as 4.1.0 instead of falling back to 4.0.0.

Add/keep coverage with MavenITgh11715EffectivePomNamespaceTest to verify the effective POM header contains:

• xmlns http://maven.apache.org/POM/4.1.0
• schemaLocation .../maven-4.1.0.xsd

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[gnodet]

Claude Code on behalf of Guillaume Nodet

The core fix in MavenXpp3Writer.configureDelegate() correctly derives the namespace and schema location from the model version, mirroring the existing pattern in TransformerSupport.write(). The fallback chain (modelVersion -> namespaceUri -> default 4.0.0) is sensible. Tests and integration test cover the primary scenario well.

Two observations:

1. Two concerns in one PR: The XmlService classloader fallback change is a separate fix from the namespace preservation. Consider splitting it into its own PR/commit so each can be reviewed and reverted independently.

2. Minor thread-safety note: configureDelegate() mutates the shared delegate field before calling write(). This is not thread-safe if a single MavenXpp3Writer instance is used concurrently. This is a pre-existing issue (same as setFileComment/setStringFormatter), but worth noting since configureDelegate is now called on every write.

[gnodet]

The version extraction from the namespace URI is a nice defensive fallback, but the constants here duplicate values already defined in TransformerSupport (NAMESPACE_FORMAT, SCHEMA_LOCATION_FORMAT). Consider extracting them to a shared location to avoid drift.

[gnodet]

This classloader fallback is a worthwhile robustness improvement, but it addresses a different problem than #11715 (namespace preservation). Consider splitting this into a separate commit/PR so the two fixes can be tracked and reverted independently.

Also, the order here (class classloader first, then TCCL) inverts the typical Java convention where TCCL is checked first. Is this intentional? In most ServiceLoader patterns the TCCL takes precedence because it represents the application's view. If the class's own classloader already sees the provider, the TCCL fallback will never trigger in practice -- which is fine for robustness, but worth documenting the intent.