Extract IO errors from h2 for streaming retries of Connection Reset (#15675)

Our streaming retries were missing connection reset errors as h2 was
shadowing IO errors (https://github.com/hyperium/h2/issues/862).

**Test plan**

In one terminal:

```
cargo python uninstall 3.12 && cargo run python install 3.12 -vv
```

In another:

```
sudo tcpkill -i wlp2s0 port 443
```

Output:

```
error: Failed to install cpython-3.12.11-linux-x86_64-gnu
  Caused by: Request failed after 3 retries
  Caused by: Failed to download https://github.com/astral-sh/python-build-standalone/releases/download/20250902/cpython-3.12.11%2B20250902-x86_64-unknown-linux-gnu-install_only_stripped.tar.gz
  Caused by: error sending request for url (https://github.com/astral-sh/python-build-standalone/releases/download/20250902/cpython-3.12.11%2B20250902-x86_64-unknown-linux-gnu-install_only_stripped.tar.gz)
  Caused by: client error (SendRequest)
  Caused by: connection error
  Caused by: connection reset
```

I don't know how to test that from inside Rust.

Fix #14171 (again, hopefully)

Cargo.lock

@@ -5380,6 +5380,7 @@ dependencies = [
  "bytecheck",
  "fs-err",
  "futures",
+ "h2",
  "html-escape",
  "http",
  "http-body-util",

crates/uv-client/Cargo.toml

@@ -38,6 +38,7 @@ async_zip = { workspace = true }
 bytecheck = { workspace = true }
 fs-err = { workspace = true, features = ["tokio"] }
 futures = { workspace = true }
+h2 = { workspace = true }
 html-escape = { workspace = true }
 http = { workspace = true }
 itertools = { workspace = true }

crates/uv-client/src/base_client.rs

@@ -983,7 +983,7 @@ impl RetryableStrategy for UvRetryableStrategy {
 /// Whether the error looks like a network error that should be retried.
 ///
 /// There are two cases that the default retry strategy is missing:
-/// * Inside the reqwest middleware error is an `io::Error` such as a broken pipe
+/// * Inside the reqwest or reqwest-middleware error is an `io::Error` such as a broken pipe
 /// * When streaming a response, a reqwest error may be hidden several layers behind errors
 ///   of different crates processing the stream, including `io::Error` layers.
 pub fn is_transient_network_error(err: &(dyn Error + 'static)) -> bool {
@@ -1027,7 +1027,12 @@ pub fn is_transient_network_error(err: &(dyn Error + 'static)) -> bool {
             }
 
             trace!("Cannot retry nested reqwest error");
-        } else if let Some(io_err) = source.downcast_ref::<io::Error>() {
+        } else if let Some(io_err) = source.downcast_ref::<io::Error>().or_else(|| {
+            // h2 may hide an IO error inside.
+            source
+                .downcast_ref::<h2::Error>()
+                .and_then(|err| err.get_io())
+        }) {
             has_known_error = true;
             let retryable_io_err_kinds = [
                 // https://github.com/astral-sh/uv/issues/12054