Restore RemotePackSync plugin identity

src/main/java/dev/lexian/remotepacksync/RemoteSha1Fetcher.java

@@ -0,0 +1,47 @@
+package dev.lexian.remotepacksync;
+
+import java.io.IOException;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.time.Duration;
+import java.util.HexFormat;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/** Downloads and validates a SHA-1 checksum without downloading the resource pack itself. */
+public final class RemoteSha1Fetcher {
+    private static final Pattern SHA1_PATTERN = Pattern.compile("(?i)(?<![0-9a-f])[0-9a-f]{40}(?![0-9a-f])");
+    private static final Duration REQUEST_TIMEOUT = Duration.ofSeconds(15);
+    private static final int MAX_RESPONSE_BYTES = 64 * 1024;
+
+    private final HttpClient client = HttpClient.newBuilder()
+            .connectTimeout(Duration.ofSeconds(10))
+            .followRedirects(HttpClient.Redirect.NORMAL)
+            .build();
+
+    public byte[] fetch(URI sha1Url) throws IOException, InterruptedException {
+        HttpRequest request = HttpRequest.newBuilder(sha1Url)
+                .GET()
+                .timeout(REQUEST_TIMEOUT)
+                .header("Accept", "text/plain, text/*;q=0.9, */*;q=0.1")
+                .header("User-Agent", "RemotePackSync/v1.0.0")
+                .build();
+
+        HttpResponse<byte[]> response = client.send(request, HttpResponse.BodyHandlers.ofByteArray());
+        if (response.statusCode() < 200 || response.statusCode() >= 300) {
+            throw new IOException("SHA-1 endpoint returned HTTP " + response.statusCode());
+        }
+        if (response.body().length > MAX_RESPONSE_BYTES) {
+            throw new IOException("SHA-1 response exceeds " + MAX_RESPONSE_BYTES + " bytes");
+        }
+
+        String responseText = new String(response.body(), java.nio.charset.StandardCharsets.UTF_8);
+        Matcher matcher = SHA1_PATTERN.matcher(responseText);
+        if (!matcher.find()) {
+            throw new IOException("SHA-1 response does not contain a 40-character hexadecimal SHA-1");
+        }
+        return HexFormat.of().parseHex(matcher.group());
+    }
+}