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 ./...

[mergify]

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[loglapa]

@Mergifyio queue

[mergify]

queue

☑️ Command disallowed due to command restrictions in the Mergify configuration.

Details

• sender-permission >= write

[loglapa]

The branch is current with main and the PR is approved. The remaining build failure occurs in CI setup before this test runs: golangci-lint panics because a dependency file requires Go 1.27 while the application was built with Go 1.26. I attempted both a failed-job rerun and the merge-queue command; GitHub restricts reruns to repository admins and Mergify restricts queueing to users with write permission. A maintainer rerun/queue after the runner toolchain is aligned is the remaining action.