restore original byte-at-a-time read in send_cmd_from_array ack loop

The ack polling loop has a tight timing requirement with
delay_microseconds_safe(1450) between iterations. Snapshotting
available() once could leave partial ack response bytes unread
until after the delay, potentially breaking cold boot timing
on some ld2420 units. Keep batch reads only in loop().

esphome/components/ld2420/ld2420.cpp

@@ -335,9 +335,10 @@ void LD2420Component::revert_config_action() {
 
 void LD2420Component::loop() {
   // If there is a active send command do not process it here, the send command call will handle it.
-  while (!this->cmd_active_ && this->available()) {
-    this->readline_(this->read(), this->buffer_data_, MAX_LINE_LENGTH);
+  if (this->cmd_active_) {
+    return;
   }
+  this->read_batch_(this->buffer_data_);
 }
 
 void LD2420Component::update_radar_data(uint16_t const *gate_energy, uint8_t sample_number) {
@@ -539,6 +540,27 @@ void LD2420Component::handle_simple_mode_(const uint8_t *inbuf, int len) {
   }
 }
 
+void LD2420Component::read_batch_(std::span<uint8_t, MAX_LINE_LENGTH> buffer) {
+  int avail = this->available();
+  if (avail == 0) {
+    return;
+  }
+
+  // Read all available bytes in batches to reduce UART call overhead.
+  uint8_t buf[MAX_LINE_LENGTH];
+  while (avail > 0) {
+    size_t to_read = std::min(static_cast<size_t>(avail), sizeof(buf));
+    if (!this->read_array(buf, to_read)) {
+      break;
+    }
+    avail -= to_read;
+
+    for (size_t i = 0; i < to_read; i++) {
+      this->readline_(buf[i], buffer.data(), buffer.size());
+    }
+  }
+}
+
 void LD2420Component::handle_ack_data_(uint8_t *buffer, int len) {
   this->cmd_reply_.command = buffer[CMD_FRAME_COMMAND];
   this->cmd_reply_.length = buffer[CMD_FRAME_DATA_LENGTH];

esphome/components/ld2420/ld2420.h

@@ -4,6 +4,7 @@
 #include "esphome/components/uart/uart.h"
 #include "esphome/core/automation.h"
 #include "esphome/core/helpers.h"
+#include <span>
 #ifdef USE_TEXT_SENSOR
 #include "esphome/components/text_sensor/text_sensor.h"
 #endif
@@ -165,6 +166,7 @@ class LD2420Component : public Component, public uart::UARTDevice {
   void handle_energy_mode_(uint8_t *buffer, int len);
   void handle_ack_data_(uint8_t *buffer, int len);
   void readline_(int rx_data, uint8_t *buffer, int len);
+  void read_batch_(std::span<uint8_t, MAX_LINE_LENGTH> buffer);
   void set_calibration_(bool state) { this->calibration_ = state; };
   bool get_calibration_() { return this->calibration_; };