[Feature]: Add getters to Atom\Link, Media\Content, and Media\Thumbnail #3

Problem

Most value objects expose getters (e.g. Image, Encoded::getContent(), Creator::getName()), but Atom\Link, Media\Content, and Media\Thumbnail only have setters. Once built, their values can't be read back, which makes it hard to test or inspect extensions returned by Item::getExtensions().

Proposed solution

Add read-only getters mirroring the existing setters:

• Link: getHref(), getRel(), getType(), getHreflang(), getTitle(), getLength()
• Thumbnail: getUrl(), getWidth(), getHeight(), getTime()
• Content: getUrl(), getFileSize(), getType(), getMedium(), getExpression(), getBitrate(), getDuration(), getWidth(), getHeight(), getLang(), getTitle(), getDescription(), getThumbnails()

Purely additive, no output change. I'll add tests and update the docs and CHANGELOG.

Question: Content::isDefault() is already the setter. Is getIsDefault() fine for the getter, or do you prefer another name?

Happy to open a PR against 8.5.x.

Alternatives considered

Parsing the generated XML or using reflection, both fragile.