[logger] Make tx_buffer_ compile-time sized

Replace the heap-allocated tx_buffer_ (new char[]) with a compile-time
sized inline array using ESPHOME_LOGGER_TX_BUFFER_SIZE define. This
eliminates a heap allocation during Logger construction, saving ~513
bytes of heap on memory-constrained devices like BK72xx.

Changes:
- Add ESPHOME_LOGGER_TX_BUFFER_SIZE define set from Python config
- Change tx_buffer_ from char* to char[ESPHOME_LOGGER_TX_BUFFER_SIZE+1]
- Remove tx_buffer_size_ member field
- Update constructor to no longer take tx_buffer_size parameter
- Update all references to use the compile-time constant

tests/dummy_main.cpp

@@ -13,7 +13,7 @@ using namespace esphome;
 
 void setup() {
   App.pre_setup("livingroom", "LivingRoom", false);
-  auto *log = new logger::Logger(115200, 512);  // NOLINT
+  auto *log = new logger::Logger(115200);  // NOLINT
   log->pre_setup();
   log->set_uart_selection(logger::UART_SELECTION_UART0);
   App.register_component(log);