Remove len==0 guard from parse(), add debug assert

All callers guarantee len > 0 (decode loop checks ptr < end,
header parse checks minimum bytes). Replace runtime check with
ESPHOME_DEBUG_API assert. This removes the len check from the
inline entirely, saving one branch per call site.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

esphome/components/api/proto.cpp

@@ -21,8 +21,6 @@ void ProtoWriteBuffer::encode_varint_raw_slow_(uint32_t value) {
 }
 
 ProtoVarIntResult ProtoVarInt::parse_slow(const uint8_t *buffer, uint32_t len) {
-  if (len == 0)
-    return {0, 0};
   // Multi-byte varint: first byte already checked to have high bit set
   uint32_t result32 = buffer[0] & 0x7F;
 #ifdef USE_API_VARINT64

esphome/components/api/proto.h

@@ -130,13 +130,15 @@ class ProtoVarInt {
   ProtoVarInt() : value_(0) {}
   explicit ProtoVarInt(uint64_t value) : value_(value) {}
 
-  /// Parse a varint from buffer. Returns result with consumed=0 on failure.
+  /// Parse a varint from buffer. Caller must ensure len >= 1.
+  /// Returns result with consumed=0 on failure (truncated multi-byte varint).
   static inline ProtoVarIntResult ESPHOME_ALWAYS_INLINE parse(const uint8_t *buffer, uint32_t len) {
+#ifdef ESPHOME_DEBUG_API
+    assert(len > 0);  // All callers guarantee len > 0
+#endif
     // Fast path: single-byte varints (0-127) are the most common case
     // (booleans, small enums, field tags, small message sizes/types).
-    // len==0 check is folded into the condition to minimize inline size;
-    // parse_slow() handles len==0.
-    if (len != 0 && (buffer[0] & 0x80) == 0) [[likely]]
+    if ((buffer[0] & 0x80) == 0) [[likely]]
       return {buffer[0], 1};
     return parse_slow(buffer, len);
   }