[sui-tool] limit sui-tool download concurrency (#24450)

## Description 

attempting to mitigate increased errors with:
```
Generic HTTP error: error decoding response body

Caused by:
    0: error decoding response body
    1: request or response body error
    2: error reading a body from connection
    3: peer closed connection without sending TLS close_notify: https://docs.rs/rustls/latest/rustls/manual/_03_howto/index.html#unexpected-eof
exit_code=1
failed at Wed Nov 26 03:17:20 PM UTC 2025
```

## Test plan 

How did you test the new or updated feature?

---

## Release notes

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your changes, release notes aren't required.

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that describes the impact of your changes that a user might notice and
any actions they must take to implement updates.

- [ ] Protocol: 
- [ ] Nodes (Validators and Full nodes): 
- [ ] gRPC:
- [ ] JSON-RPC: 
- [ ] GraphQL: 
- [ ] CLI: 
- [ ] Rust SDK:
- [ ] Indexing Framework:

Author: johnjmartin

crates/sui-tool/src/commands.rs

@@ -227,8 +227,7 @@ pub enum ToolCommand {
         /// skip downloading indexes dir
         #[clap(long = "skip-indexes")]
         skip_indexes: bool,
-        /// Number of parallel downloads to perform. Defaults to a reasonable
-        /// value based on number of available logical cores.
+        /// Number of parallel downloads to perform. Defaults to 50, max 200.
         #[clap(long = "num-parallel-downloads")]
         num_parallel_downloads: Option<usize>,
         /// Network to download snapshot for. Defaults to "mainnet".
@@ -293,8 +292,7 @@ pub enum ToolCommand {
         genesis: PathBuf,
         #[clap(long = "path")]
         path: PathBuf,
-        /// Number of parallel downloads to perform. Defaults to a reasonable
-        /// value based on number of available logical cores.
+        /// Number of parallel downloads to perform. Defaults to 50, max 200.
         #[clap(long = "num-parallel-downloads")]
         num_parallel_downloads: Option<usize>,
         /// Verification mode to employ.
@@ -691,11 +689,7 @@ impl ToolCommand {
                         .update_log("off")
                         .expect("Failed to update log level");
                 }
-                let num_parallel_downloads = num_parallel_downloads.unwrap_or_else(|| {
-                    num_cpus::get()
-                        .checked_sub(1)
-                        .expect("Failed to get number of CPUs")
-                });
+                let num_parallel_downloads = num_parallel_downloads.unwrap_or(50).min(200);
                 let snapshot_bucket =
                     snapshot_bucket.or_else(|| match (network, no_sign_request) {
                         (Chain::Mainnet, false) => Some(
@@ -846,11 +840,7 @@ impl ToolCommand {
                         .update_log("off")
                         .expect("Failed to update log level");
                 }
-                let num_parallel_downloads = num_parallel_downloads.unwrap_or_else(|| {
-                    num_cpus::get()
-                        .checked_sub(1)
-                        .expect("Failed to get number of CPUs")
-                });
+                let num_parallel_downloads = num_parallel_downloads.unwrap_or(50).min(200);
                 let snapshot_bucket =
                     snapshot_bucket.or_else(|| match (network, no_sign_request) {
                         (Chain::Mainnet, false) => Some(

crates/sui-tool/src/lib.rs

@@ -1056,9 +1056,13 @@ async fn backfill_epoch_transaction_digests(
         }
     });
 
+    // Use reduced concurrency for backfill to avoid overwhelming the remote server
+    // when running in parallel with snapshot download
+    let backfill_concurrency = (concurrency / 4).max(1);
+
     futures::stream::iter(checkpoints_to_fetch)
         .map(|sq| CheckpointReader::fetch_from_object_store(&client, sq))
-        .buffer_unordered(concurrency)
+        .buffer_unordered(backfill_concurrency)
         .try_for_each(|checkpoint| {
             let perpetual_db = perpetual_db.clone();
             let tx_counter = tx_counter.clone();