test: share the photo payload fixture, drop two test-only aliases

The one-photo MediaItemPayload literal was written out at six sites across
the urls and send test modules; ctx::test_support::photo_item holds it once.
permanent_error now delegates to api_error (it stays a fn pointer because
that is what MockSender::scripted takes), misskey's x_media_site_id wrapper
is gone in favour of the function it renamed, and PixivAPI is no longer
re-exported -- nothing outside pixiv/api.rs names it.

crates/x-media/src/site/misskey/interface.rs

@@ -220,11 +220,7 @@ mod tests {
             cache_key("https://misskey.io/notes/aotihl10lqrs015s"),
             Some("misskey:aotihl10lqrs015s".to_string())
         );
-        assert_eq!(x_media_site_id("misskey:abc"), "misskey");
-    }
-
-    fn x_media_site_id(key: &str) -> &'static str {
-        crate::site::site_id_from_key(key)
+        assert_eq!(crate::site::site_id_from_key("misskey:abc"), "misskey");
     }
 
     #[test]

crates/x-media/src/site/pixiv/mod.rs

@@ -2,7 +2,7 @@ mod api;
 mod interface;
 mod model;
 
-pub use api::{PixivAPI, PixivError, disable, fetch, validate};
+pub use api::{PixivError, disable, fetch, validate};
 pub use interface::{
     Illustration, PATTERN, PixivSite, cache_key, enabled, fetch_from_url, is_retryable,
     media_headers,