fix: use default retriever router

[mahoushoujyo-eee]

Fixes #954

Summary

• use the resolved default router when Config.Router is nil
• add a regression test for the default all-retrievers routing behavior

Tests

• go test ./flow/retriever/router
• go test ./flow/retriever/...
• golangci-lint run ./flow/retriever/router --timeout 5m

Note: go test ./... currently has unrelated Windows-local failures in ADK filesystem/plantask/reduction tests and a CRLF/path assertion. golangci-lint run ./... --timeout 5m reports unrelated existing gci issues in files not touched by this PR.

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[Copilot]

Pull request overview

Fixes a nil-pointer panic in the retriever router by ensuring the resolved (default) router function is stored on the constructed routerRetriever when Config.Router is not provided.

Changes:

• Store the resolved router (default or user-provided) in routerRetriever instead of config.Router.
• Add a regression test verifying the default routing behavior calls all registered retrievers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
flow/retriever/router/router.go	Fixes the nil router assignment by storing the resolved default router function.
flow/retriever/router/router_test.go	Adds a regression test to ensure the default router queries all configured retrievers.

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