[api] Fix review comments and add USE_API guards

- Update comments to reflect that ProtoService methods moved to
  APIConnection, not APIServerConnectionBase
- Fix comment referring to read_message as "override"
- Wrap #include "api_connection.h" and read_message_ implementation
  in #ifdef USE_API guards

esphome/components/api/api_connection.h

@@ -55,8 +55,8 @@ class APIConnection final : public APIServerConnectionBase {
   void loop();
 
  protected:
-  // Override read_message here (instead of in APIServerConnectionBase) so the
-  // compiler can devirtualize and inline on_* handler calls within this class.
+  // read_message_ is defined here (instead of in APIServerConnectionBase) so the
+  // compiler can devirtualize and inline on_* handler calls within this final class.
   void read_message_(uint32_t msg_size, uint32_t msg_type, const uint8_t *msg_data);
 
   // Auth helpers defined here (not in ProtoService) so the compiler can

esphome/components/api/api_pb2_service.cpp

@@ -1,7 +1,9 @@
 // This file was automatically generated with a tool.
 // See script/api_protobuf/api_protobuf.py
 #include "api_pb2_service.h"
+#ifdef USE_API
 #include "api_connection.h"
+#endif
 #include "esphome/core/log.h"
 
 namespace esphome::api {
@@ -21,6 +23,7 @@ void APIServerConnectionBase::log_receive_message_(const LogString *name) {
 }
 #endif
 
+#ifdef USE_API
 void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const uint8_t *msg_data) {
   // Check authentication/connection requirements
   switch (msg_type) {
@@ -706,5 +709,6 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       break;
   }
 }
+#endif  // USE_API
 
 }  // namespace esphome::api

esphome/components/api/proto.h

@@ -697,7 +697,7 @@ inline void ProtoLengthDelimited::decode_to_message(ProtoDecodableMessage &msg)
 
 template<typename T> const char *proto_enum_to_string(T value);
 
-// ProtoService removed — all methods moved to APIServerConnectionBase.
-// APIConnection is the only concrete class; virtual dispatch was unnecessary.
+// ProtoService removed — its methods were inlined into APIConnection.
+// APIConnection is the concrete server-side implementation; the extra virtual layer was unnecessary.
 
 }  // namespace esphome::api

script/api_protobuf/api_protobuf.py

@@ -2947,7 +2947,9 @@ namespace esphome::api {
     cpp = FILE_HEADER
     cpp += """\
 #include "api_pb2_service.h"
+#ifdef USE_API
 #include "api_connection.h"
+#endif
 #include "esphome/core/log.h"
 
 namespace esphome::api {
@@ -3051,11 +3053,12 @@ static const char *const TAG = "api.service";
                 result += "#endif\n"
         return result
 
-    # Generate read_message as APIConnection method (not base class) so the compiler
+    # Generate read_message_ as APIConnection method (not base class) so the compiler
     # can devirtualize and inline the on_* handler calls within the same class.
-    # APIConnection declares the override in api_connection.h.
+    # APIConnection declares this method in api_connection.h.
 
-    out = "void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const uint8_t *msg_data) {\n"
+    out = "#ifdef USE_API\n"
+    out += "void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const uint8_t *msg_data) {\n"
 
     # Auth check block before dispatch switch
     out += "  // Check authentication/connection requirements\n"
@@ -3100,6 +3103,7 @@ static const char *const TAG = "api.service";
     out += "      break;\n"
     out += "  }\n"
     out += "}\n"
+    out += "#endif  // USE_API\n"
     cpp += out
     hpp += "};\n"