[web_server_idf] Move session adoption to out-of-line cold path

Keep the hot path in AsyncEventSource::loop() to the atomic load,
branch, and dead-session cleanup. On a real connect, call a
noinline+cold helper (adopt_pending_sessions_main_loop_) to swap the
pending list out, drop any already-disconnected sessions, and invoke
on_connect_ / start_session_main_loop_ on the rest.

esphome/components/web_server_idf/web_server_idf.cpp

@@ -493,26 +493,9 @@ void AsyncEventSource::handleRequest(AsyncWebServerRequest *request) {
 }
 
 bool AsyncEventSource::loop() {
-  // Fast path: one atomic load per tick. Lock only on a real connect.
+  // Fast path: one atomic load per tick. Slow path is out-of-line on connect.
   if (this->has_pending_sessions_.load(std::memory_order_acquire)) {
-    std::vector<AsyncEventSourceResponse *> incoming;
-    {
-      LockGuard guard{this->pending_mutex_};
-      incoming.swap(this->pending_sessions_);
-      this->has_pending_sessions_.store(false, std::memory_order_relaxed);
-    }
-    for (auto *rsp : incoming) {
-      // Already disconnected? Drop it; skip on_connect_/prime on a dead session.
-      if (rsp->fd_.load() == 0) {
-        delete rsp;  // NOLINT(cppcoreguidelines-owning-memory)
-        continue;
-      }
-      this->sessions_.push_back(rsp);
-      if (this->on_connect_) {
-        this->on_connect_(rsp);
-      }
-      rsp->start_session_main_loop_();
-    }
+    this->adopt_pending_sessions_main_loop_();
   }
 
   // Clean up dead sessions safely
@@ -535,6 +518,27 @@ bool AsyncEventSource::loop() {
   return !this->sessions_.empty();
 }
 
+void AsyncEventSource::adopt_pending_sessions_main_loop_() {
+  std::vector<AsyncEventSourceResponse *> incoming;
+  {
+    LockGuard guard{this->pending_mutex_};
+    incoming.swap(this->pending_sessions_);
+    this->has_pending_sessions_.store(false, std::memory_order_relaxed);
+  }
+  for (auto *rsp : incoming) {
+    // Already disconnected? Drop it; skip on_connect_/session start on a dead session.
+    if (rsp->fd_.load() == 0) {
+      delete rsp;  // NOLINT(cppcoreguidelines-owning-memory)
+      continue;
+    }
+    this->sessions_.push_back(rsp);
+    if (this->on_connect_) {
+      this->on_connect_(rsp);
+    }
+    rsp->start_session_main_loop_();
+  }
+}
+
 void AsyncEventSource::try_send_nodefer(const char *message, const char *event, uint32_t id, uint32_t reconnect) {
   for (auto *ses : this->sessions_) {
     if (ses->fd_.load() != 0) {  // Skip dead sessions

esphome/components/web_server_idf/web_server_idf.h

@@ -350,6 +350,9 @@ class AsyncEventSource : public AsyncWebHandler {
   size_t count() const { return this->sessions_.size(); }
 
  protected:
+  // Cold path: move sessions from pending_sessions_ into sessions_ and greet each one.
+  void __attribute__((noinline, cold)) adopt_pending_sessions_main_loop_();
+
   std::string url_;
   // Main-loop only. Vector: SSE sessions are 1-5 connections, linear search beats set.
   std::vector<AsyncEventSourceResponse *> sessions_;