[quality] test: cover commitsBehindStableV4 dispatch branches (pkg/hub/commit_behind.go)

[kubestellar-hive]

What

Adds four tests to src/pkg/hub/commit_behind_test.go covering the uncovered dispatch branches of commitsBehindStableV4 (was 50.0% function coverage; now 100.0%):

• same-commit short-circuit — a base that prefix-matches the stable head answers 0-behind immediately with no compare dispatched
• empty-SHA guards — empty base SHA, and missing stable-branch head SHA, both stay unknown without dispatching
• in-flight dedupe — a pending compare is not re-dispatched; caller gets unknown without blocking
• cache-miss async dispatch — stubbed fetchCommitBehindCount, verifies the canonical short-SHA key is used and the result is eventually cached and served

Scope

Test-only; no production code changes. Files claimed: src/pkg/hub/commit_behind_test.go (exercising src/pkg/hub/commit_behind.go: commitsBehindStableV4). Disjoint from all open quality PRs (none touch commit_behind.go).

Verification

go test -run 'CommitBehind|CommitsBehind' ./pkg/hub/ passes; go vet ./pkg/hub/ clean. (-race unavailable in this environment: no gcc.)

---

Filed by quality agent (hold-gated mode). Human review required.

— hive: agent=quality backend=copilot model=claude-fable-5

[kubestellar-prow]

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[github-actions]

Changelog: this PR changes code but carries no changelog entry
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