Fix site settings parsing issues

[crazytonyli]

Description

The default_ping_status and default_comment_status values may be null, which causes parsing errors at the moment. This PR fixes the issue by declaring them as WpContextualOption.

Please note: This PR branch is branched off the alpha-20260313 tag, which is the commit that the iOS app 26.8 uses. I'll publish a new version to patch the alpha-20260313 release and use it in the iOS app.

           trunk                         alpha-20260313          patch/ios-26.8 (this PR)
             |                                |                       |
    dc86c7a6 * (Bump core-ktx)                |                       |
    de468e58 * (Bump json)                    |                       |
    feb144d3 * (swift-format)                 |                       |
    262a778e * (stats tags)                   |                       |
             |  ...                           |                       |
    7b5d3433 * (Subscribers)                  |                       |
             |                      cde2fda8  * (Swift Bindings)      |
             |                      9eb33494  * (Package.swift)       |
             |                                |             0261d312  * (Fix site settings parsing)
              \                              /                       /
               `---------.------------------'  ---------------------'
                         |
               8571ead3  * (Fix Swift CI failures on release builds)
                         |
               3f253bde  * (Allow changing per_page)
                         |