test(p2p): provide tail in partial range store

[loglapa]

Follow-up to #406.

handleRangeRequest now queries Tail before checking the requested range. The focused partialRangeStore test double embedded a nil header.Store, so its inherited Tail method panicked and made the current main test suite fail deterministically.

Give the test double an explicit tail at the requested lower bound and implement Tail, preserving the original test path: the range is not below tail, HasAt remains false, and the assertion still verifies that to is not expanded to the distant head.

Validation

• go test ./...
• go vet ./...
• golangci-lint run
• go test ./p2p -run ^'TestExchangeServer_partialRangeNotExpanded$' -count=100\n- go test -race ./p2p -run ^'TestExchangeServer_partialRangeNotExpanded$' -count=20\n- docker run --rm -v "$PWD:/workspace" -w /workspace golang:1.27 go test ./...