Use HrefList only for multiple hrefs

[rfc2822]

• Owner allows only one href
• HrefListProperty: remove shortcut to get first value because all values should be processed (shortcut encourages unclean processing)
• remove deprecated UrlUtils.equals()

[Copilot]

Pull Request Overview

This PR updates how href properties are processed by using HrefList only for multiple href elements and removing deprecated shortcuts and helpers.

• Updated tests to reflect that only a single href is used from the provided list.
• Removed the shortcut “href” from HrefListProperty and replaced webdav imports with common imports.
• Removed the deprecated UrlUtils.equals() method and updated the XmlReader logic to capture only the first matching tag.

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/test/kotlin/at/bitfire/dav4jvm/property/OwnerTest.kt	Added tests for empty, multiple, and invalid owner hrefs
src/main/kotlin/at/bitfire/dav4jvm/property/webdav/Owner.kt	Switched from extending HrefListProperty to using a dedicated href property
src/main/kotlin/at/bitfire/dav4jvm/property/common/HrefListProperty.kt	Removed the first-element shortcut and updated package and API usage
Other property files (GroupMembership, AddressbookHomeSet, Source, CalendarUserAddressSet, CalendarProxyWriteFor, CalendarProxyReadFor, CalendarHomeSet)	Updated imports to reference the common HrefListProperty
src/main/kotlin/at/bitfire/dav4jvm/XmlReader.kt	Adjusted tag processing to capture only the first matching entry
src/main/kotlin/at/bitfire/dav4jvm/UrlUtils.kt	Removed the deprecated equals() method

Comments suppressed due to low confidence (2)

src/main/kotlin/at/bitfire/dav4jvm/XmlReader.kt:93

• It would be helpful to document that the extra condition 'result == null' is purposely used to capture only the first matching tag, ensuring consistent processing of multiple href elements.

            if (eventType == XmlPullParser.START_TAG && parser.depth == depth + 1 && parser.propertyName() == name && result == null)

src/main/kotlin/at/bitfire/dav4jvm/property/common/HrefListProperty.kt:24

• Since the shortcut accessor 'href' was removed in favor of processing all href values separately, please consider adding a comment clarifying this change for future maintainability.

abstract class HrefListProperty(