Address review feedback

- Avoid unnecessary vector construction in static mode path
- Use wraparound-safe millis comparison for staged RX
- Fix stale comments (20ms -> 40ms, ESP-IDF -> ESP32)
- Clarify MODBUS_BITS_PER_CHAR is approximate

esphome/components/modbus/modbus.cpp

@@ -11,7 +11,7 @@ static const char *const TAG = "modbus";
 // Maximum bytes to log for Modbus frames (truncated if larger)
 static constexpr size_t MODBUS_MAX_LOG_BYTES = 64;
 
-// Bits per character on the wire: 1 start + 8 data + 1 parity + 1 stop
+// Approximate bits per character on the wire (depends on parity/stop bit config)
 static constexpr uint32_t MODBUS_BITS_PER_CHAR = 11;
 // Milliseconds per second
 static constexpr uint32_t MS_PER_SEC = 1000;

esphome/components/uart/uart_component.h

@@ -189,7 +189,7 @@ class UARTComponent {
   InternalGPIOPin *rx_pin_{};
   InternalGPIOPin *flow_control_pin_{};
   size_t rx_buffer_size_{};
-  // ESP-IDF always sets this at codegen time via set_rx_full_threshold().
+  // ESP32 (both Arduino and ESP-IDF) always sets this at codegen time via set_rx_full_threshold().
   // Other platforms (USB UART, Arduino, etc.) leave it unset.
   size_t rx_full_threshold_{RX_FULL_THRESHOLD_UNSET};
   size_t rx_timeout_{0};

tests/integration/fixtures/external_components/uart_mock/automation.h

@@ -27,8 +27,8 @@ template<typename... Ts> class MockUartInjectRXAction : public Action<Ts...>, pu
   void play(const Ts &...x) override {
     if (this->len_ >= 0) {
       // Static mode: use pointer and length
-      std::vector<uint8_t> data(this->code_.data, this->code_.data + this->len_);
       if (this->delay_ms_ > 0) {
+        std::vector<uint8_t> data(this->code_.data, this->code_.data + this->len_);
         this->parent_->inject_to_rx_buffer_delayed(data, this->delay_ms_);
       } else {
         this->parent_->inject_to_rx_buffer(this->code_.data, static_cast<size_t>(this->len_));

tests/integration/fixtures/external_components/uart_mock/uart_mock.cpp

@@ -54,7 +54,8 @@ void MockUartComponent::loop() {
   }
 
   // Process staged RX - deliver bytes whose delay has elapsed
-  while (!this->staged_rx_.empty() && millis() >= this->staged_rx_.front().available_at_ms) {
+  uint32_t now_ms = millis();
+  while (!this->staged_rx_.empty() && (static_cast<int32_t>(now_ms - this->staged_rx_.front().available_at_ms) >= 0)) {
     auto &staged = this->staged_rx_.front();
     ESP_LOGD(TAG, "Delivering %zu staged RX bytes", staged.data.size());
     this->inject_to_rx_buffer(staged.data);

tests/integration/fixtures/uart_mock_modbus_no_threshold.yaml

@@ -38,7 +38,7 @@ uart_mock:
                     0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
                     0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
                     0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00};
-                - uart_mock.inject_rx: # Second USB packet: rest of response (staged with 20ms latency)
+                - uart_mock.inject_rx: # Second USB packet: rest of response (staged with 40ms latency)
                     delay: 40ms
                     data: !lambda return{0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00,
                       0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x42,0x6F,0xCC,0xCD,0x43,0x7C,0xB8,0x10,0x3D,0x38,0x51,0xEC,

tests/integration/test_uart_mock_modbus.py

@@ -232,7 +232,7 @@ async def test_uart_mock_modbus_no_threshold(
 ) -> None:
     """Test modbus with no rx_full_threshold (simulating USB UART).
 
-    Without the 50ms fallback timeout, the chunked response with a 20ms gap
+    Without the 50ms fallback timeout, the chunked response with a 40ms gap
     between USB packets would cause a false timeout and CRC failure cascade.
     """
     # Replace external component path placeholder