test: share the handler/cache fixtures from ctx::test_support

The same fixtures were rebuilt in five test modules: a `CachedPost`
literal in `link_cache.rs`, `handlers/urls.rs` and twice in
`send/mod.rs`, the edit-before-forward prompt in `handlers/mod.rs` and
`handlers/callback.rs`, and a scripted API error in both handler
modules. They now live in `ctx::test_support` next to `TestStores`:

- `cached_photo()` — the canonical cached post (photo + file id at
  `https://x.com/u/status/1`, key `twitter:1`); tests mutate the fields
  they care about, as the caption-quote test already did.
- `seed_prompt(template, created_at)` + `PROMPT_ID`/`FORWARDED_ID` —
  the prompt record, the chat template and the bound forward channel.
  The two former copies differed only in which knob the caller set (the
  callback tests backdate it for the expiry cases, the reply tests pick
  the template), so the union is one helper.
- `api_error(message)` — construction only; each test module keeps its
  own message constant, because the wording is what that module's path
  answers with (`chat not found` vs `message not found`).

`send/mod.rs`'s `cached_sequence_cache_data()` (which re-extracted the
post out of the task it had just built) is gone: the two settle tests
seed the cache from the same builder the task uses.

No behaviour change: the values are the ones the tests used except
`file_id` (`AgAC-file-id` everywhere, asserted in the link-cache
round-trip) and `sensitive` (the unasserted `true` in the link-cache
fixture), and every test still passes unchanged.

Verified: `cargo fmt`, `cargo clippy --workspace --all-targets --locked
-- -D warnings` and `cargo test --workspace --locked` (180 passed, 14
ignored).

crates/xmedia-bot/src/link_cache.rs

@@ -180,23 +180,7 @@ impl LinkCache {
 #[cfg(test)]
 mod tests {
     use super::*;
-
-    fn entry() -> CachedPost {
-        CachedPost {
-            url: "https://x.com/u/status/1".into(),
-            caption: "cap".into(),
-            title: "t".into(),
-            content: "c".into(),
-            author: "a".into(),
-            author_url: "au".into(),
-            tags: "".into(),
-            sensitive: true,
-            media: vec![CachedMedia {
-                kind: CachedMediaKind::Photo,
-                file_id: "AgAC...".into(),
-            }],
-        }
-    }
+    use crate::ctx::test_support::cached_photo;
 
     /// A payload written before the title/content split has no `content`
     /// field. It must still read back — the cache deletes what it cannot
@@ -247,12 +231,12 @@ mod tests {
         let cache = LinkCache::new(
             crate::db::open_store(dir.path().join("c.db").to_str().unwrap()).unwrap(),
         );
-        cache.put("twitter:1", &entry()).await;
+        cache.put("twitter:1", &cached_photo()).await;
         let got = cache.get("twitter:1", Duration::from_secs(3600)).await;
         assert!(got.is_some());
         let got = got.unwrap();
         assert_eq!(got.url, "https://x.com/u/status/1");
-        assert_eq!(got.media[0].file_id, "AgAC...");
+        assert_eq!(got.media[0].file_id, "AgAC-file-id");
     }
 
     #[tokio::test]
@@ -261,7 +245,7 @@ mod tests {
         let cache = LinkCache::new(
             crate::db::open_store(dir.path().join("c.db").to_str().unwrap()).unwrap(),
         );
-        cache.put("twitter:1", &entry()).await;
+        cache.put("twitter:1", &cached_photo()).await;
         // Force the row into the past so a 1s TTL expires it.
         {
             let conn = rusqlite::Connection::open(dir.path().join("c.db")).unwrap();
@@ -314,8 +298,8 @@ mod tests {
         let cache = LinkCache::new(
             crate::db::open_store(dir.path().join("c.db").to_str().unwrap()).unwrap(),
         );
-        cache.put("twitter:1", &entry()).await;
-        cache.put("pixiv:2", &entry()).await;
+        cache.put("twitter:1", &cached_photo()).await;
+        cache.put("pixiv:2", &cached_photo()).await;
         cache.remove("twitter:1").await;
         assert!(
             cache
@@ -349,8 +333,8 @@ mod tests {
         let cache = LinkCache::new(
             crate::db::open_store(dir.path().join("c.db").to_str().unwrap()).unwrap(),
         );
-        cache.put("twitter:1", &entry()).await;
-        cache.put("pixiv:2", &entry()).await;
+        cache.put("twitter:1", &cached_photo()).await;
+        cache.put("pixiv:2", &cached_photo()).await;
         // By key: only the matching row is removed.
         assert_eq!(cache.clear(Some("twitter:1")).await, 1);
         assert!(