Inline empty() fast path, out-of-line drain+error handling

Keep the cheap overflow_buf_.empty() check inline at all call sites.
Move try_drain + HELPER_LOG + error handling into a single out-of-line
drain_overflow_and_handle_errors_() used by both write_raw_ and the
subclass loop() methods. Eliminates the duplicated drain logic.

esphome/components/api/api_frame_helper.cpp

@@ -100,6 +100,15 @@ const LogString *api_error_to_logstr(APIError err) {
   return LOG_STR("UNKNOWN");
 }
 
+APIError APIFrameHelper::drain_overflow_and_handle_errors_() {
+  if (this->overflow_buf_.try_drain(this->socket_.get()) == -1) {
+    HELPER_LOG("Socket write failed with errno %d", errno);
+    if (this->check_socket_write_err_(errno) != APIError::WOULD_BLOCK)
+      return APIError::SOCKET_WRITE_FAILED;
+  }
+  return APIError::OK;
+}
+
 // Write data to socket, overflow to backlog buffer if LWIP TCP send buffer is full.
 // Returns OK if all data was sent or successfully queued.
 // Returns SOCKET_WRITE_FAILED on hard error (sets state to FAILED).
@@ -114,11 +123,9 @@ APIError APIFrameHelper::write_raw_(const struct iovec *iov, int iovcnt, uint16_
 
   // Drain any existing backlog first
   if (!this->overflow_buf_.empty()) [[unlikely]] {
-    if (this->overflow_buf_.try_drain(this->socket_.get()) == -1) {
-      HELPER_LOG("Socket write failed with errno %d", errno);
-      if (this->check_socket_write_err_(errno) != APIError::WOULD_BLOCK)
-        return APIError::SOCKET_WRITE_FAILED;
-    }
+    APIError err = this->drain_overflow_and_handle_errors_();
+    if (err != APIError::OK)
+      return err;
   }
 
   // If backlog is clear, try direct send

esphome/components/api/api_frame_helper.h

@@ -190,15 +190,11 @@ class APIFrameHelper {
   }
 
  protected:
-  // Drain any backlogged overflow data to the socket.
-  // Returns OK even for WOULD_BLOCK to avoid connection termination.
-  APIError try_drain_overflow_buffer_() {
-    if (!this->overflow_buf_.empty() && this->overflow_buf_.try_drain(this->socket_.get()) == -1) {
-      if (this->check_socket_write_err_(errno) != APIError::WOULD_BLOCK)
-        return APIError::SOCKET_WRITE_FAILED;
-    }
-    return APIError::OK;
-  }
+  // Drain backlogged overflow data to the socket and handle errors.
+  // Called when overflow_buf_.empty() is false. Out-of-line to keep the
+  // fast path (empty check) inline at call sites.
+  // Returns OK for transient errors (WOULD_BLOCK), SOCKET_WRITE_FAILED for hard errors.
+  APIError drain_overflow_and_handle_errors_();
 
   // Common implementation for writing raw data to socket
   APIError write_raw_(const struct iovec *iov, int iovcnt, uint16_t total_write_len);

esphome/components/api/api_frame_helper_noise.cpp

@@ -153,11 +153,12 @@ APIError APINoiseFrameHelper::loop() {
     }
   }
 
-  APIError err = this->try_drain_overflow_buffer_();
-  if (err != APIError::OK) {
-    HELPER_LOG("Overflow drain failed with errno %d", errno);
+  if (!this->overflow_buf_.empty()) [[unlikely]] {
+    APIError err = this->drain_overflow_and_handle_errors_();
+    if (err != APIError::OK)
+      return err;
   }
-  return err;
+  return APIError::OK;
 }
 
 /** Read a packet into the rx_buf_.

esphome/components/api/api_frame_helper_plaintext.cpp

@@ -64,11 +64,12 @@ APIError APIPlaintextFrameHelper::loop() {
   if (state_ != State::DATA) {
     return APIError::BAD_STATE;
   }
-  APIError err = this->try_drain_overflow_buffer_();
-  if (err != APIError::OK) {
-    HELPER_LOG("Overflow drain failed with errno %d", errno);
+  if (!this->overflow_buf_.empty()) [[unlikely]] {
+    APIError err = this->drain_overflow_and_handle_errors_();
+    if (err != APIError::OK)
+      return err;
   }
-  return err;
+  return APIError::OK;
 }
 
 /** Read a packet into the rx_buf_.