[combination] Use FixedVector for sensor sources to eliminate realloc machinery

The source count is known at config time, so pre-allocate with
FixedVector::init() instead of using std::vector which pulls in
_M_realloc_insert template instantiation code.

esphome/components/combination/combination.h

@@ -1,10 +1,10 @@
 #pragma once
 
 #include "esphome/core/component.h"
+#include "esphome/core/helpers.h"
 #include "esphome/components/sensor/sensor.h"
 
 #include <functional>
-#include <vector>
 
 namespace esphome {
 namespace combination {
@@ -42,6 +42,7 @@ class CombinationNoParameterComponent : public CombinationComponent {
 // Base class for opertions that require one parameter to compute the combination
 class CombinationOneParameterComponent : public CombinationComponent {
  public:
+  void set_source_count(size_t count) { this->sensor_sources_.init(count); }
   void add_source(Sensor *sensor, std::function<float(float)> const &compute);
   void add_source(Sensor *sensor, float value);
 
@@ -55,7 +56,7 @@ class CombinationOneParameterComponent : public CombinationComponent {
   };
 
  protected:
-  std::vector<SensorSource> sensor_sources_;
+  FixedVector<SensorSource> sensor_sources_;
 };
 
 class KalmanCombinationComponent : public CombinationOneParameterComponent {

esphome/components/combination/sensor.py

@@ -180,6 +180,9 @@ async def to_code(config):
     if proces_std_dev := config.get(CONF_PROCESS_STD_DEV):
         cg.add(var.set_process_std_dev(proces_std_dev))
 
+    if config[CONF_TYPE] in (CONF_KALMAN, CONF_LINEAR):
+        cg.add(var.set_source_count(len(config[CONF_SOURCES])))
+
     for source_conf in config[CONF_SOURCES]:
         source = await cg.get_variable(source_conf[CONF_SOURCE])
         if config[CONF_TYPE] == CONF_KALMAN: