core: transfer long key encoding buffer (#13628)

The long-preedit fallback introduced in e95b1707c intentionally
allocated twice. The encoder wrote into an oversized caller-owned buffer
and returned only the written subslice, so transferring it required
manually shrinking the allocation or tracking its original capacity. The
copy kept that rare path simple.

The key encoder moved to std.Io.Writer.Allocating in 44496df899. Its
toOwnedSlice method handles shrinking and ownership transfer, remapping
when the allocator supports it and falling back to an allocation and
copy when it does not. Use it directly for WriteReq.alloc to remove the
guaranteed second allocation while preserving cleanup on failure.

src/Surface.zig

@@ -3244,17 +3244,15 @@ fn encodeKey(
         );
         defer alloc_writer.deinit();
 
-        // This results in a double allocation but this is such an unlikely
-        // path the performance impact is unimportant.
         try input.key_encode.encode(
             &alloc_writer.writer,
             event,
             encoding_opts,
         );
-        break :req try termio.Message.WriteReq.init(
-            self.alloc,
-            alloc_writer.writer.buffered(),
-        );
+        break :req .{ .alloc = .{
+            .alloc = self.alloc,
+            .data = try alloc_writer.toOwnedSlice(),
+        } };
     };
 
     // Copy the encoded data into the inspector event if we have one.