fix(server,agent): apply Tasks 3-5 review fixes (non-blocking)

From the secure-session-core Tasks 3-5 code review (APPROVE-WITH-FIXES):
- MEDIUM-2: delete the dead `validate_agent_key` "accept-any-key" placeholder +
  its AuthenticatedAgent/AuthState scaffolding (zero callers; the real agent
  auth is validate_agent_api_key + per-agent cak_ keys). Removes an auth landmine.
- LOW-3: stop interpolating support-code values into 3 relay log lines (bearer
  credentials).
- LOW-1: document the X-Real-IP trust requirement in ip_extract.rs (NPM must set
  it from $remote_addr); behavior unchanged.
- LOW-2: correct the consent/heartbeat comment in agent session loop (the loop
  awaits the dialog; safe because CONSENT_TIMEOUT 60s < HEARTBEAT_TIMEOUT 90s).
cargo fmt/clippy(-D warnings)/test all green on GURU-5070 (89 tests, 0 warnings).
MEDIUM-1 (viewer-token logout revocation) remains a tracked follow-up.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/relay/mod.rs

@@ -561,7 +561,7 @@ async fn handle_agent_connection(
     if let Some(ref code) = support_code {
         // Check if the code is cancelled or invalid
         if support_codes.is_cancelled(code).await {
-            warn!("Agent tried to connect with cancelled code: {}", code);
+            warn!("Agent tried to connect with a cancelled support code");
             // Send disconnect message to agent
             let disconnect_msg = proto::Message {
                 payload: Some(proto::message::Payload::Disconnect(proto::Disconnect {
@@ -740,7 +740,7 @@ async fn handle_agent_connection(
             interval.tick().await;
             if let Some(ref code) = support_code_check {
                 if support_codes_check.is_cancelled(code).await {
-                    info!("Support code {} was cancelled, disconnecting agent", code);
+                    info!("Support code was cancelled, disconnecting agent");
                     // Send disconnect message
                     let disconnect_msg = proto::Message {
                         payload: Some(proto::message::Payload::Disconnect(proto::Disconnect {
@@ -917,7 +917,7 @@ async fn handle_agent_connection(
                 let _ = db::support_codes::mark_code_completed(db.pool(), code).await;
             }
 
-            info!("Support code {} marked as completed", code);
+            info!("Support code marked as completed");
         }
     }