[uart] Change available() return type from int to size_t (#13893)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

esphome/components/uart/uart_component_rp2040.cpp

@@ -186,7 +186,7 @@ bool RP2040UartComponent::read_array(uint8_t *data, size_t len) {
 #endif
   return true;
 }
-int RP2040UartComponent::available() { return this->serial_->available(); }
+size_t RP2040UartComponent::available() { return this->serial_->available(); }
 void RP2040UartComponent::flush() {
   ESP_LOGVV(TAG, "    Flushing");
   this->serial_->flush();