test(A.1 #8): cover ResumeUnsupported restart path in transcription downloader

The downloader already handles servers that return 200 to a Range
request by falling through to a truncating File::create on the .part
path, discarding stale partial bytes. That branch had no dedicated
fixture test — the SHA mismatch and Range-honouring resume cases were
covered but the mirror / proxy that strips Range support was not.

Adds spawn_no_range_server (always 200, full body regardless of Range
header) and download_file_restarts_when_server_ignores_range. Writes 12
bytes of stale content to .part, kicks off a download, asserts the
final file matches the fresh body exactly (not stale-bytes-prefixed)
and the .part file is cleaned up.

Brings the transcription downloader to test-coverage parity with
crates/llm/src/model_manager.rs per brief item #8 ("test coverage
parity" acceptance).

crates/transcription/src/model_manager.rs

@@ -350,6 +350,30 @@ mod tests {
         addr
     }
 
+    /// A minimal HTTP server that always responds with 200 + the full
+    /// body, even when the request carries a `Range` header. Mirrors a
+    /// mirror / proxy that strips Range support, exercising the
+    /// ResumeUnsupported branch in `download_file`.
+    async fn spawn_no_range_server(content: Vec<u8>) -> std::net::SocketAddr {
+        let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+
+        tokio::spawn(async move {
+            let (mut socket, _) = listener.accept().await.unwrap();
+            let mut buf = vec![0u8; 2048];
+            let _ = socket.read(&mut buf).await.unwrap();
+            let response = format!(
+                "HTTP/1.1 200 OK\r\n\
+                 Content-Length: {}\r\n\r\n",
+                content.len(),
+            );
+            socket.write_all(response.as_bytes()).await.unwrap();
+            socket.write_all(&content).await.unwrap();
+        });
+
+        addr
+    }
+
     /// ModelFile stores `&'static str` fields, so we leak the strings
     /// once per test — tests are one-shot, so the cost is noise.
     fn leak(s: String) -> &'static str {
@@ -387,6 +411,42 @@ mod tests {
         assert_eq!(sha256_of_file(&dest).unwrap(), expected_sha);
     }
 
+    #[tokio::test]
+    async fn download_file_restarts_when_server_ignores_range() {
+        // Covers the ResumeUnsupported branch documented in `download_file`:
+        // when a partial `.part` file exists and the server returns 200
+        // (full body) to our Range request, we must discard the stale
+        // partial bytes and write the fresh body from offset zero rather
+        // than appending on top.
+        let body = b"fresh transcription payload that replaces any stale partial".to_vec();
+        let addr = spawn_no_range_server(body.clone()).await;
+
+        let dir = tempdir().unwrap();
+        let dest = dir.path().join("fixture.bin");
+        let part = dest.with_extension("bin.part");
+        // Pretend a previous attempt downloaded 12 bytes of something
+        // entirely unrelated. If the client naively appended the 200
+        // body, the final file would start with these bytes.
+        std::fs::write(&part, b"STALE_BYTES1").unwrap();
+
+        let file = ModelFile {
+            filename: leak(dest.file_name().unwrap().to_string_lossy().into_owned()),
+            url: leak(format!("http://{addr}/fixture.bin")),
+            size: kon_core::types::Megabytes(0),
+            sha256: None,
+        };
+        let id = ModelId::new("test-fixture");
+
+        download_file(&file, &dest, &id, &|_| ()).await.unwrap();
+
+        let bytes = std::fs::read(&dest).unwrap();
+        assert_eq!(
+            bytes, body,
+            "server returned 200 to Range — downloader must discard stale .part and rewrite from scratch"
+        );
+        assert!(!part.exists(), ".part → dest rename must run after restart");
+    }
+
     #[tokio::test]
     async fn download_file_fails_on_sha_mismatch_and_cleans_part_file() {
         let body = b"speech-to-text fixture body".to_vec();