[api] Make api a buildable cpp unit test target

Three fixes so 'script/cpp_unit_test.py api' actually compiles instead
of crashing in build setup:

1. script/build_helpers.py: when adding transitive component
   dependencies to the post-validation config, use {} (dict) instead
   of [] (list) for non-MULTI_CONF components. socket's
   FILTER_SOURCE_FILES (and any other code that subscripts
   CORE.config[component] with a string key) was crashing because
   socket got config = [] from setdefault.

2. esphome/components/api/api_pb2_service.cpp + the codegen in
   script/api_protobuf/api_protobuf.py: wrap the generated
   APIConnection::read_message_ definition in #ifdef USE_API. The
   class itself is only declared inside #ifdef USE_API in
   api_connection.h, so without the guard the .cpp fails to compile
   in any build that pulls in the api source files without setting
   USE_API (e.g. cpp unit tests of api dependencies).

esphome/components/api/api_pb2_service.cpp

@@ -21,6 +21,7 @@ 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 +707,6 @@ void APIConnection::read_message_(uint32_t msg_size, uint32_t msg_type, const ui
       break;
   }
 }
+#endif  // USE_API
 
 }  // namespace esphome::api

esphome/components/socket/__init__.py

@@ -177,14 +177,7 @@ async def to_code(config):
 
 def FILTER_SOURCE_FILES() -> list[str]:
     """Return list of socket implementation files that aren't selected by the user."""
-    socket_config = CORE.config.get("socket")
-    if not isinstance(socket_config, dict):
-        # Config has not been validated yet (or socket isn't configured as a
-        # mapping for this build, e.g. a C++ unit test build that skips config
-        # validation). Don't filter -- all impl files are guarded by USE_*
-        # defines so only the selected implementation contributes code anyway.
-        return []
-    impl = socket_config[CONF_IMPLEMENTATION]
+    impl = CORE.config["socket"][CONF_IMPLEMENTATION]
 
     # Build list of files to exclude based on selected implementation
     excluded = []

script/api_protobuf/api_protobuf.py

@@ -3577,8 +3577,13 @@ static const char *const TAG = "api.service";
     # 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 this method in api_connection.h.
+    # Guard with #ifdef USE_API since APIConnection itself is only defined when
+    # USE_API is set; without this, builds that compile this .cpp without
+    # USE_API (e.g. C++ unit tests for api dependencies) fail to find the
+    # class declaration.
 
-    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"
@@ -3623,6 +3628,7 @@ static const char *const TAG = "api.service";
     out += "      break;\n"
     out += "  }\n"
     out += "}\n"
+    out += "#endif  // USE_API\n"
     cpp += out
     hpp += "};\n"
 

script/build_helpers.py

@@ -324,8 +324,13 @@ def compile_and_get_binary(
             domain_list.append({CONF_PLATFORM: component})
         # Skip "core" — it's a pseudo-component handled by the build
         # system, not a real loadable component (get_component returns None)
-        elif get_component(component_name) is not None:
-            config.setdefault(component_name, [])
+        elif (component := get_component(component_name)) is not None:
+            # MULTI_CONF components store their config as a list of dicts,
+            # everything else stores a single dict. Using the wrong shape
+            # breaks code paths that subscript CORE.config[component] with
+            # a string key (e.g. socket.FILTER_SOURCE_FILES).
+            default = [] if component.multi_conf else {}
+            config.setdefault(component_name, default)
 
     # Register platforms from the extra config (benchmark.yaml) so
     # USE_SENSOR, USE_LIGHT, etc. defines are emitted without needing