[spi] Enable host-platform builds for unit testing (#15188)

esphome/components/spi/spi.cpp

@@ -68,7 +68,7 @@ void SPIComponent::dump_config() {
   LOG_PIN("  SDI Pin: ", this->sdi_pin_);
   LOG_PIN("  SDO Pin: ", this->sdo_pin_);
   for (size_t i = 0; i != this->data_pins_.size(); i++) {
-    ESP_LOGCONFIG(TAG, "  Data pin %u: GPIO%d", i, this->data_pins_[i]);
+    ESP_LOGCONFIG(TAG, "  Data pin %zu: GPIO%d", i, this->data_pins_[i]);
   }
   if (this->spi_bus_->is_hw()) {
     ESP_LOGCONFIG(TAG, "  Using HW SPI: %s", this->interface_name_);
@@ -118,4 +118,12 @@ uint16_t SPIDelegateBitBash::transfer_(uint16_t data, size_t num_bits) {
   return out_data;
 }
 
+#if !defined(USE_ESP32) && !defined(USE_ARDUINO)
+// Stub for unsupported platforms (host, Zephyr, etc.) - hardware SPI is unavailable
+SPIBus *SPIComponent::get_bus(SPIInterface interface, GPIOPin *clk, GPIOPin *sdo, GPIOPin *sdi,
+                              const std::vector<uint8_t> &data_pins) {
+  return nullptr;
+}
+#endif
+
 }  // namespace esphome::spi

esphome/components/spi/spi.h

@@ -23,9 +23,9 @@ using SPIInterface = SPIClassRP2040 *;
 using SPIInterface = SPIClass *;
 #endif
 
-#elif defined(CLANG_TIDY)
+#elif defined(USE_HOST) || defined(CLANG_TIDY)
 
-using SPIInterface = void *;  // Stub for platforms without SPI (e.g., Zephyr)
+using SPIInterface = void *;  // Stub for platforms without SPI (e.g., host, Zephyr)
 
 #endif  // USE_ESP32 / USE_ARDUINO