fix: use resolved router variable instead of config.Router in routerRetriever

[kuishou68]

Summary

Fixes a nil pointer dereference bug in flow/retriever/router/router.go.

Problem

In NewRetriever, a local router variable is correctly resolved to a default function when config.Router is nil. However, the struct literal then stores config.Router (still nil) instead of the local router variable. This means calling Retrieve() without a custom Router configured panics at runtime:

runtime error: invalid memory address or nil pointer dereference

Fix

Change the struct initialization to use the resolved router variable:

// Before (buggy)
return &routerRetriever{
    retrievers: config.Retrievers,
    router:     config.Router,  // nil when Router not provided
    fusionFunc: fusion,
}, nil

// After (fixed)
return &routerRetriever{
    retrievers: config.Retrievers,
    router:     router,  // always non-nil: either config.Router or the default
    fusionFunc: fusion,
}, nil

Closes #954

[CLAassistant]

All committers have signed the CLA.