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/xmedia-bot/src/rate_limit.rs

@@ -91,6 +91,15 @@ impl TokenBucket {
         tokio::time::sleep(Duration::from_secs_f64(wait)).await;
     }
 
+    /// Current balance, for the tests that assert a call site charged the
+    /// bucket (a charge is otherwise only observable as a delay).
+    #[cfg(test)]
+    pub(crate) fn tokens(&self) -> f64 {
+        let mut state = self.state.lock();
+        self.refill(&mut state);
+        state.tokens
+    }
+
     /// True when the bucket has refilled to capacity: no debt outstanding, so
     /// the chat has not sent anything recently.
     fn is_idle(&self) -> bool {