fix(recorder): propagate context, handle short writes, add nil guards, idempotent Close, and doc comments

- httpRecorder: use http.NewRequestWithContext instead of http.NewRequest
- tcpRecorder: loop Write to avoid short-write data loss
- fileRecorder: guard against nil out in Record and Close
- plugin/grpc, plugin/http: handle json.Marshal errors instead of ignoring
- fileRecorder, redis*Recorder: idempotent Close via sync.Once
- All exported symbols: add doc comments

recorder/plugin/grpc.go

@@ -55,7 +55,10 @@ func (p *grpcPlugin) Record(ctx context.Context, b []byte, opts ...recorder.Reco
 		opt(&options)
 	}
 
-	md, _ := json.Marshal(options.Metadata)
+	md, err := json.Marshal(options.Metadata)
+	if err != nil {
+		return err
+	}
 
 	reply, err := p.client.Record(ctx,
 		&proto.RecordRequest{

recorder/plugin/http.go

@@ -56,7 +56,10 @@ func (p *httpPlugin) Record(ctx context.Context, b []byte, opts ...recorder.Reco
 		opt(&options)
 	}
 
-	md, _ := json.Marshal(options.Metadata)
+	md, err := json.Marshal(options.Metadata)
+	if err != nil {
+		return err
+	}
 
 	rb := httpPluginRequest{
 		Data:     b,