test: drive a real Bot against a stand-in API

Every test went through `MockSender`, so `media_sender`'s `Bot`
implementation — the URL it builds, the multipart it sends, the per-chat
limiter and the bot-wide budget it charges — was never exercised, and neither
was any handler reached from a real update. The two things that made that hard
are gone:

- `media_sender::test_support::fake_api::FakeApi` is a stand-in for
  `api.telegram.org`: a `tokio` TCP listener that reads one HTTP/1.1 request
  (JSON or multipart), records it and answers the smallest result the method
  needs. No new dependency, and `Bot::new(token).set_api_url(api.url())`
  points a real `Bot` at it. Note for future tests: teloxide keys methods by
  payload type, so the path is `SendMediaGroup`, not `sendMediaGroup`.
- `message_handler` built its own `AppContext::from_statics` internally, so no
  test could reach its branches; its body is now `handle_message(ctx, bot,
  message)` with `message_handler` as the thin `dptree` entry.

Tests: a media group through the real `Bot` (asserting the multipart fields —
chat, media URL, caption — and that the send charged the chat's limiter), the
forward button through the real callback path (`CopyMessages`,
`DeleteMessage`, `AnswerCallbackQuery` with the prompt's ids and the toast
text), and `handle_message` twice (a prompt reply becoming an
`EditMessageCaption`, and a supported link in a group producing the one
explanatory `SendMessage`).

Also closes the redirect-hop gap left open by the download guard: the live
`a_redirect_into_the_hosts_network_is_refused` follows a public redirector to
`169.254.169.254` and asserts the policy refuses the hop (verified against
httpbin.org here, and by mutation — disabling the hop check fails it).

Docs: AGENTS.md's testing conventions and untested-modules list (the Bot
implementation and the handler branches are covered now; `main.rs`'s
startup/shutdown and its `dptree` tree still are not).

`cargo fmt`, `cargo clippy --workspace --all-targets --locked -- -D
warnings`, `cargo test --workspace --locked` (201 passed, 16 ignored) clean.

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

@@ -956,6 +956,21 @@ mod tests {
         }
     }
 
+    /// The redirect-hop guard, against a public redirector: the initial URL is
+    /// checked by [`media_request`], but a redirect is the part of the path a
+    /// third-party response actually controls.
+    #[tokio::test]
+    #[ignore = "live network: requires outbound HTTPS to httpbin.org"]
+    async fn a_redirect_into_the_hosts_network_is_refused() {
+        let url = "https://httpbin.org/redirect-to?url=http://169.254.169.254/latest/meta-data/";
+        match download_media(url).await.unwrap_err() {
+            // A policy refusal reaches the caller wrapped by reqwest.
+            FetchError::Http(e) => assert!(e.is_redirect(), "got {e}"),
+            FetchError::Blocked => {}
+            other => panic!("expected a refusal, got {other:?}"),
+        }
+    }
+
     #[tokio::test]
     async fn a_download_into_the_hosts_network_is_refused() {
         // Refused on the URL alone: nothing has to be listening (or leaking) at