Fix ODR violation: generate api_pb2_defines.h for consistent ProtoVarInt layout

USE_API_VARINT64 was only defined in api_pb2.h, but proto.cpp (where
decode() and parse_wide() live) includes proto.h directly. This caused:

1. parse_wide() not compiled in proto.cpp (guarded by #ifdef)
2. decode() used 32-bit-only parse(), returning {} for varints > 5 bytes
3. ODR violation: ProtoVarInt was 4 bytes in proto.cpp but 8 bytes in
   api_pb2*.cpp translation units

For bluetooth_proxy, 48-bit BLE addresses encode as 7-byte varints.
The failed parse caused decode() to return early, leaving request_type
at its default value of 0 (BLUETOOTH_DEVICE_REQUEST_TYPE_CONNECT),
producing spurious "V1 connections removed" errors.

Fix: Generate api_pb2_defines.h with the USE_API_VARINT64 define and
include it from proto.h, ensuring all translation units see a consistent
ProtoVarInt layout.

esphome/components/api/api_pb2.h

@@ -2,10 +2,6 @@
 // See script/api_protobuf/api_protobuf.py
 #pragma once
 
-#include "esphome/core/defines.h"
-#ifdef USE_BLUETOOTH_PROXY
-#define USE_API_VARINT64
-#endif
 #include "esphome/core/string_ref.h"
 
 #include "proto.h"

esphome/components/api/api_pb2_defines.h

@@ -0,0 +1,8 @@
+// This file was automatically generated with a tool.
+// See script/api_protobuf/api_protobuf.py
+#pragma once
+
+#include "esphome/core/defines.h"
+#ifdef USE_BLUETOOTH_PROXY
+#define USE_API_VARINT64
+#endif

esphome/components/api/proto.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include "api_pb2_defines.h"
 #include "esphome/core/component.h"
 #include "esphome/core/helpers.h"
 #include "esphome/core/log.h"

script/api_protobuf/api_protobuf.py

@@ -2593,22 +2593,28 @@ def main() -> None:
     )
 
     # Find the ifdef guard for 64-bit varint fields (int64/uint64/sint64).
-    # Emitted before proto.h so parse_wide_() and 64-bit accessors are available.
+    # Generated into api_pb2_defines.h so proto.h can include it, ensuring
+    # consistent ProtoVarInt layout across all translation units.
     has_varint64, varint64_guard = get_varint64_ifdef(file, message_ifdef_map)
 
+    # Generate api_pb2_defines.h — included by proto.h to ensure all translation
+    # units see USE_API_VARINT64 consistently (avoids ODR violations in ProtoVarInt).
+    defines_content = FILE_HEADER
+    defines_content += "#pragma once\n\n"
+    defines_content += '#include "esphome/core/defines.h"\n'
+    if has_varint64:
+        defines_content += "\n".join(
+            wrap_with_ifdef(["#define USE_API_VARINT64"], varint64_guard)
+        )
+        defines_content += "\n"
+
+    with open(root / "api_pb2_defines.h", "w", encoding="utf-8") as f:
+        f.write(defines_content)
+
     content = FILE_HEADER
     content += """\
 #pragma once
 
-#include "esphome/core/defines.h"
-"""
-    if has_varint64:
-        content += "\n".join(
-            wrap_with_ifdef(["#define USE_API_VARINT64"], varint64_guard)
-        )
-        content += "\n"
-
-    content += """\
 #include "esphome/core/string_ref.h"
 
 #include "proto.h"