[api] Restore single-buffer write_raw_ overload to keep inline failure path small

Moving iovec construction into write_raw_fast_ caused an 8% regression
by enlarging the inlined failure path. The 33-byte out-of-line wrapper
keeps the hot inline path minimal.

esphome/components/api/api_frame_helper.cpp

@@ -112,8 +112,15 @@ APIError APIFrameHelper::drain_overflow_and_handle_errors_() {
   return APIError::OK;
 }
 
-// Slow path: handles partial writes, errors, and overflow buffering.
-// Called when the inline fast path in the header couldn't complete the write.
+// Single-buffer write path: wraps in iovec and delegates.
+APIError APIFrameHelper::write_raw_(const void *data, uint16_t len, ssize_t sent) {
+  struct iovec iov = {const_cast<void *>(data), len};
+  return this->write_raw_(&iov, 1, len, sent);
+}
+
+// Handles partial writes, errors, and overflow buffering.
+// Called when the inline fast path in the header couldn't complete the write,
+// or directly from cold paths (handshake, error handling).
 // sent == -1 means either the fast path write returned -1, or there was overflow backlog.
 APIError APIFrameHelper::write_raw_(const struct iovec *iov, int iovcnt, uint16_t total_write_len, ssize_t sent) {
 #ifdef HELPER_LOG_PACKETS

esphome/components/api/api_frame_helper.h

@@ -202,8 +202,7 @@ class APIFrameHelper {
       if (sent == static_cast<ssize_t>(len)) [[likely]]
         return APIError::OK;
     }
-    struct iovec iov = {const_cast<void *>(data), len};
-    return this->write_raw_(&iov, 1, len, sent);
+    return this->write_raw_(data, len, sent);
   }
   inline APIError ESPHOME_ALWAYS_INLINE write_raw_fast_(const struct iovec *iov, int iovcnt, uint16_t total_write_len) {
     ssize_t sent = -1;
@@ -215,7 +214,8 @@ class APIFrameHelper {
     return this->write_raw_(iov, iovcnt, total_write_len, sent);
   }
 
-  // Slow path (out-of-line): handle partial writes, errors, overflow buffering
+  // Out-of-line write paths: handle partial writes, errors, overflow buffering
+  APIError write_raw_(const void *data, uint16_t len, ssize_t sent = -1);
   APIError write_raw_(const struct iovec *iov, int iovcnt, uint16_t total_write_len, ssize_t sent = -1);
 
   // Socket ownership (4 bytes on 32-bit, 8 bytes on 64-bit)

esphome/components/api/api_frame_helper_noise.cpp

@@ -537,8 +537,7 @@ APIError APINoiseFrameHelper::write_frame_(const uint8_t *data, uint16_t len) {
   header[2] = (uint8_t) len;
 
   if (len == 0) {
-    struct iovec iov = {header, 3};
-    return this->write_raw_(&iov, 1, 3);
+    return this->write_raw_(header, 3);
   }
   struct iovec iov[2];
   iov[0].iov_base = header;

esphome/components/api/api_frame_helper_plaintext.cpp

@@ -219,13 +219,11 @@ APIError APIPlaintextFrameHelper::read_packet(ReadPacketBuffer *buffer) {
                                                 "Bad indicator byte";
       char msg[INDICATOR_MSG_SIZE];
       memcpy_P(msg, MSG_PROGMEM, INDICATOR_MSG_SIZE);
-      struct iovec iov = {msg, INDICATOR_MSG_SIZE};
-      this->write_raw_(&iov, 1, INDICATOR_MSG_SIZE);
+      this->write_raw_(msg, INDICATOR_MSG_SIZE);
 #else
       static const char MSG[] = "\x00"
                                 "Bad indicator byte";
-      struct iovec iov = {const_cast<char *>(MSG), INDICATOR_MSG_SIZE};
-      this->write_raw_(&iov, 1, INDICATOR_MSG_SIZE);
+      this->write_raw_(MSG, INDICATOR_MSG_SIZE);
 #endif
     }
     return aerr;