Fix #892: ignore whitespace-only unnamed XML text as unknown property

[arimu1]

Fixes #892

Root cause

Pretty-printed XML with attribute-only beans/records exposes whitespace-only element body as the unnamed XML text property ("" by default). With FAIL_ON_UNKNOWN_PROPERTIES that synthetic property is treated as unknown, even though it is only formatting whitespace between the start and end tags.

This is the same unnamed-text channel used for @JacksonXmlText ([dataformat-xml#565]); it should fail only when the text is actual unknown content.

Change

• Override XmlDeserializationContext.handleUnknownProperty to skip the unnamed text property when its value is whitespace-only.
• Mapped @JacksonXmlText is unchanged (the property is known).
• Non-whitespace unnamed text still fails when the property is unknown.

Test plan

• WhitespaceOnlyUnknownProp892Test — issue XML on records + POJO; non-whitespace body still UnrecognizedPropertyException
• Neighbors: XmlTextWhitespace565Test, EmptyBeanDeser318Test, XmlEmptyRecordDeser508Test, XmlRecordDeser735Test, XmlClassDeser735Test, TextValueTest, XmlTextTest
• Full mvn test on JDK 21

AI-assisted (Composer 2.5); human-reviewed.