test: truncate attribute string values using Unicode rune count (#8219)

Add test cases to ensure correct behavior.

From https://opentelemetry.io/docs/specs/otel/common/#attribute-limits:

> set an attribute value length limit such that for each attribute
value:
> - if it is a string, if it exceeds that limit (counting any character
in it as 1), SDKs MUST truncate that value, so that its length is at
most equal to the limit,

sdk/trace/span_test.go

@@ -200,6 +200,27 @@ func TestTruncateAttr(t *testing.T) {
 			attr:  strSliceAttr,
 			want:  strSliceAttr,
 		},
+		{
+			// Multi-byte string: byte length (9) exceeds limit (5) but rune count (3) does not.
+			// Must not be truncated.
+			limit: 5,
+			attr:  attribute.String(key, "日本語"),
+			want:  attribute.String(key, "日本語"),
+		},
+		{
+			// Multi-byte string: both byte length and rune count exceed limit.
+			// Must be truncated to limit runes.
+			limit: 2,
+			attr:  attribute.String(key, "日本語"),
+			want:  attribute.String(key, "日本"),
+		},
+		{
+			// STRINGSLICE with multi-byte elements: byte lengths exceed limit but rune counts do not.
+			// Must not be truncated.
+			limit: 1,
+			attr:  attribute.StringSlice(key, []string{"日", "本"}),
+			want:  attribute.StringSlice(key, []string{"日", "本"}),
+		},
 	}
 
 	for _, test := range tests {