Fix memory leak in TEI processing: cache JAXP factories, close streams

[lfoppiano]

PR #7 fixed a leak in the PDF path by closing DocumentSource; TEI has no DocumentSource, but heap and file-descriptor usage still grew under sustained /processDatasetTEI load. Root causes:

1. JAXP factory churn. DocumentBuilderFactory.newInstance(), XPathFactory.newInstance(), TransformerFactory.newInstance(), and SAXParserFactory.newInstance() ran on every request (and, in XMLUtilities.segment(), on every sentence). Each call re-runs ServiceLoader discovery and produces factories whose classloader- backed caches are not reclaimed promptly.
2. DocumentBuilder.parse(File) defers FileInputStream closure to Xerces, accumulating FDs under sustained load.
3. The parsed DOM Document was left as a local reference until method return, delaying young-gen reclaim of a large node graph.

Fix:

• Cache factories as private static finals in DatasetParser and XMLUtilities, with synchronized accessors (newDocumentBuilder, newXPath, newSAXParser, newTransformer). Factories' new*() methods are not guaranteed thread-safe; synchronized access keeps contention negligible versus XML parse cost and avoids ThreadLocal leaks in the Dropwizard thread pool.
• Parse TEI via try-with-resources FileInputStream so the handle is released deterministically.
• Null the parsed Document reference in finally blocks to aid GC.
• Remove dead DocumentBuilderFactory allocation in processXML(File).

No PDF-path changes; PR #7's DocumentSource.close() is preserved.

[Copilot]

Pull request overview

This PR aims to stop heap and file-descriptor growth under sustained /processDatasetTEI load by reducing JAXP factory churn and ensuring XML inputs/DOMs are released more deterministically during TEI/XML processing.

Changes:

• Cache JAXP factories (DocumentBuilderFactory/XPathFactory/SAXParserFactory/TransformerFactory) and provide synchronized new*() helpers to avoid repeated ServiceLoader discovery.
• Parse TEI via explicitly managed streams (try-with-resources) to deterministically close file handles.
• Remove redundant per-call factory allocations in hot paths (notably XMLUtilities.segment() and parts of DatasetParser).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/main/java/org/grobid/core/utilities/XMLUtilities.java	Introduces cached JAXP factories and routes multiple XML/XPath/transform operations through synchronized helper constructors.
src/main/java/org/grobid/core/engines/DatasetParser.java	Caches JAXP factories, switches TEI parsing to explicit InputStream handling, and reuses cached XPath creation.

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