chore: fix clippy lints from nightly-2025-03-16 (#11273)

I like to run nightly clippy every so often to make our future rust
upgrades easier. Some notable changes:

* Prefer `next_back()` over `last()`. Generic iterators will implement
`last()` to run forward through the iterator until the end.

* Prefer `io::Error::other()`.

* Use implicit returns

One case where I haven't dealt with the issues is the now
[more-sensitive "large enum variant"
lint](https://github.com/rust-lang/rust-clippy/pull/13833). I chose not
to take any decisions around it here, and simply marked them as allow
for now.

proxy/src/cancellation.rs

@@ -425,12 +425,7 @@ impl CancelClosure {
             &mut mk_tls,
             &self.hostname,
         )
-        .map_err(|e| {
-            CancelError::IO(std::io::Error::new(
-                std::io::ErrorKind::Other,
-                e.to_string(),
-            ))
-        })?;
+        .map_err(|e| CancelError::IO(std::io::Error::other(e.to_string())))?;
 
         self.cancel_token.cancel_query_raw(socket, tls).await?;
         debug!("query was cancelled");

proxy/src/proxy/tests/mod.rs

@@ -568,7 +568,7 @@ fn helper_create_cached_node_info(cache: &'static NodeInfoCache) -> CachedNodeIn
 fn helper_create_connect_info(
     mechanism: &TestConnectMechanism,
 ) -> auth::Backend<'static, ComputeCredentials> {
-    let user_info = auth::Backend::ControlPlane(
+    auth::Backend::ControlPlane(
         MaybeOwned::Owned(ControlPlaneClient::Test(Box::new(mechanism.clone()))),
         ComputeCredentials {
             info: ComputeUserInfo {
@@ -578,8 +578,7 @@ fn helper_create_connect_info(
             },
             keys: ComputeCredentialKeys::Password("password".into()),
         },
-    );
-    user_info
+    )
 }
 
 fn config() -> ComputeConfig {

proxy/src/serverless/conn_pool_lib.rs

@@ -47,6 +47,7 @@ impl ConnInfo {
 }
 
 #[derive(Clone)]
+#[allow(clippy::large_enum_variant, reason = "TODO")]
 pub(crate) enum ClientDataEnum {
     Remote(ClientDataRemote),
     Local(ClientDataLocal),