Add optional XSD validation

[BorislavSabev]

Changelog

Added

• BaseDomBuilder::validateSchema(?string $xsdFile = null): bool — validates the generated XML against an XSD. Defaults to the ISO 20022 XSD bundled for the builder's pain format; pass a path to validate against a bank's variant XSD.
• BaseDomBuilder::getSchemaValidationErrors(?string $xsdFile = null): array — same, but returns one Line N: message string per error; empty when the document is valid.
• Passthroughs for both on BaseCustomerTransferFileFacade. They render the document first, with the same single-shot semantics as asXML().
• MessageFormat::getBundledSchemaPath(): ?string — path to the bundled XSD for a format, or null when none is bundled.
• tests/Unit/DomBuilder/SchemaValidationTest.php — 52 tests: every supported format has a bundled XSD matching the test fixture, valid SCT/SDD files pass for every pain.001.001.x / pain.008.001.x version, errors are reported, explicit paths are used, exceptions for missing XSDs, libxml state is restored, and the facade passthroughs.

Changed

• ext-libxml moved from require-dev to require; BaseDomBuilder now calls libxml_* functions.
• Docs: doc/guides/output-and-validation.md no longer claims the XSDs aren't shipped and documents the new methods; DomBuilder, facade and exceptions reference pages updated.

Notes

• No new files in the package: the XSDs are the ones already under doc/ISO20022/. A test fails if .gitattributes ever excludes that directory.
• Validation runs on a re-parsed copy of the XML. Validating asDoc() directly always fails, because the root's xmlns is set as an attribute and the in-memory elements aren't in the schema's namespace.
• Files built with setOmitAgentElementIfBicMissing(true) can fail the base ISO XSD (it drops mandatory DbtrAgt/CdtrAgt). Validate those against the bank's own XSD.
• InvalidArgumentException is thrown when the XSD path doesn't exist, or when no XSD is bundled for the format and none was passed.
• The methods aren't added to DomBuilderInterface or CustomerTransferFileFacadeInterface, so custom implementations don't break.