Address review feedback: fix unconditional guard bug, add ifndef guard, clarify comment

- Fix get_varint64_ifdef() to return unconditional guard when any 64-bit
  varint field has no ifdef (None in ifdefs set)
- Wrap USE_API_VARINT64 define with #ifndef to avoid redefinition warnings
  when esphome/core/defines.h also defines it (test/IDE builds)
- Clarify proto.h comment about uint32_t shift behavior at byte 4
- Add namespace to generated api_pb2_defines.h for linter compliance

esphome/components/api/api_pb2_defines.h

@@ -4,5 +4,9 @@
 
 #include "esphome/core/defines.h"
 #ifdef USE_BLUETOOTH_PROXY
+#ifndef USE_API_VARINT64
 #define USE_API_VARINT64
 #endif
+#endif
+
+namespace esphome::api {}  // namespace esphome::api

esphome/components/api/proto.h

@@ -119,7 +119,7 @@ class ProtoVarInt {
     }
     // 32-bit phase: process remaining bytes with native 32-bit shifts.
     // Without USE_API_VARINT64: cover bytes 1-4 (shifts 7, 14, 21, 28) — the uint32_t
-    // shift at byte 4 truncates upper bits but those are always zero for valid uint32 values.
+    // shift at byte 4 (shift by 28) may lose bits 32-34, but those are always zero for valid uint32 values.
     // With USE_API_VARINT64: cover bytes 1-3 (shifts 7, 14, 21) so parse_wide handles
     // byte 4+ with full 64-bit arithmetic (avoids truncating values > UINT32_MAX).
     uint32_t result32 = buffer[0] & 0x7F;

script/api_protobuf/api_protobuf.py

@@ -1929,6 +1929,9 @@ def get_varint64_ifdef(
     }
     if not ifdefs:
         return False, None
+    if None in ifdefs:
+        # At least one 64-bit varint field is unconditional, so the guard must be unconditional.
+        return True, None
     ifdefs.discard(None)
     return True, ifdefs.pop() if len(ifdefs) == 1 else None
 
@@ -2603,10 +2606,14 @@ def main() -> None:
     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)
-        )
+        lines = [
+            "#ifndef USE_API_VARINT64",
+            "#define USE_API_VARINT64",
+            "#endif",
+        ]
+        defines_content += "\n".join(wrap_with_ifdef(lines, varint64_guard))
         defines_content += "\n"
+    defines_content += "\nnamespace esphome::api {}  // namespace esphome::api\n"
 
     with open(root / "api_pb2_defines.h", "w", encoding="utf-8") as f:
         f.write(defines_content)