fix(agent-data-plane): preserve DNS for hostname IPC endpoints (1.3.x backport) (#2069)

## Summary

Follow-up to PR #2059. This keeps the no-DNS IPC behavior for literal-IP endpoints like `127.0.0.1`, but preserves DNS resolution when the configured IPC endpoint host is a hostname.

Also includes the small doc wording cleanup on `without_dns_resolution()`.

## Testing

- `cargo fmt --package datadog-agent-commons --package saluki-io --check`
- `cargo test -p datadog-agent-commons ipc::client::tests`

Note: full `cargo fmt --check` still hits existing generated-file formatting diffs under stable rustfmt on this branch.

Co-authored-by: jesse.szwedko <jesse.szwedko@datadoghq.com>

Author: aqian01

lib/datadog-agent/commons/src/ipc/client/mod.rs

@@ -1,6 +1,6 @@
 //! Helpers for interacting with the Datadog Agent.
 
-use std::time::Duration;
+use std::{net::IpAddr, time::Duration};
 
 use backon::Retryable as _;
 use datadog_protos::agent::v1::{RefreshRemoteAgentRequest, RegisterRemoteAgentRequest, RegisterRemoteAgentResponse};
@@ -15,7 +15,7 @@ use saluki_error::{generic_error, ErrorContext as _, GenericError};
 use saluki_io::net::client::http::HttpsCapableConnectorBuilder;
 use tonic::{
     service::interceptor::InterceptedService,
-    transport::{Channel, Endpoint},
+    transport::{Channel, Endpoint, Uri},
     Code, Request, Response,
 };
 use tracing::warn;
@@ -68,15 +68,20 @@ impl RemoteAgentClient {
             let auth_interceptor = BearerAuthInterceptor::from_file(&config.auth().auth_token_file_path()).await?;
             let ipc_cert_file_path = config.auth().ipc_cert_file_path();
             let client_tls_config = build_ipc_client_ipc_tls_config(ipc_cert_file_path).await?;
-            let connector_builder = HttpsCapableConnectorBuilder::default().without_dns_resolution();
+            let endpoint = config.endpoint()?;
+            let connector_builder = HttpsCapableConnectorBuilder::default();
+            let connector_builder = if endpoint_requires_dns_resolution(&endpoint) {
+                connector_builder
+            } else {
+                connector_builder.without_dns_resolution()
+            };
             #[cfg(target_os = "linux")]
             let connector_builder = if let Some(addr) = config.vsock_addr()? {
                 connector_builder.with_vsock_addr(addr)
             } else {
                 connector_builder
             };
             let https_connector = connector_builder.build(client_tls_config)?;
-            let endpoint = config.endpoint()?;
             let channel = Endpoint::from(endpoint.clone())
                 .connect_timeout(Duration::from_secs(2))
                 .connect_with_connector(https_connector)
@@ -240,6 +245,19 @@ impl RemoteAgentClient {
     }
 }
 
+fn endpoint_requires_dns_resolution(endpoint: &Uri) -> bool {
+    match endpoint.host() {
+        Some(host) => {
+            let host = host
+                .strip_prefix('[')
+                .and_then(|host| host.strip_suffix(']'))
+                .unwrap_or(host);
+            host.parse::<IpAddr>().is_err()
+        }
+        None => false,
+    }
+}
+
 async fn try_query_agent_api(
     client: &mut AgentSecureClient<InterceptedService<Channel, BearerAuthInterceptor>>,
 ) -> Result<(), GenericError> {
@@ -260,3 +278,25 @@ async fn try_query_agent_api(
         },
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use tonic::transport::Uri;
+
+    use super::endpoint_requires_dns_resolution;
+
+    #[test]
+    fn hostname_endpoint_requires_dns_resolution() {
+        let endpoint = "https://datadog-agent:5001".parse::<Uri>().expect("valid URI");
+        assert!(endpoint_requires_dns_resolution(&endpoint));
+    }
+
+    #[test]
+    fn literal_ip_endpoints_do_not_require_dns_resolution() {
+        let ipv4_endpoint = "https://127.0.0.1:5001".parse::<Uri>().expect("valid URI");
+        assert!(!endpoint_requires_dns_resolution(&ipv4_endpoint));
+
+        let ipv6_endpoint = "https://[::1]:5001".parse::<Uri>().expect("valid URI");
+        assert!(!endpoint_requires_dns_resolution(&ipv6_endpoint));
+    }
+}

lib/saluki-io/src/net/client/http/conn.rs

@@ -483,7 +483,7 @@ impl HttpsCapableConnectorBuilder {
     /// DNS, such as Unix sockets, vsock, or literal-IP TCP endpoints. Hostname-based TCP
     /// destinations will fail to resolve when this is enabled.
     ///
-    /// Defaults to enabled.
+    /// DNS resolution is enabled by default.
     pub fn without_dns_resolution(mut self) -> Self {
         self.dns_resolution_disabled = true;
         self