[quality] cover pkg/github ETagCacheStats and MergeVerdictKey (0% → 100%)

[hivecommons-hive]

Test Improvement

Adds hermetic unit tests for two exported pkg/github contract functions that were at 0.0% coverage on main (9d44557):

• TestETagCacheStats_ReportsSharedCache (src/pkg/github/etagcache_stats_test.go) — exercises ETagCacheStats through etagCacheWrap + the existing etagServer httptest helper: one charged fetch, one free 304 revalidation, asserts hits/misses/entries deltas of 1/1/1, and cleans its entry out of the process-wide shared cache. Proves the operator's GitHub API quota exhausted 1s after every reset: all 16 repos fail to enumerate, scheduler runs blind #7430 quota telemetry reads the cache the transport chain actually uses.
• TestMergeVerdictKey (src/pkg/github/merge_verdict_test.go) — pins the Repo + "#" + Number key spelling (bare repo, owner/name, zero value) that the governor's verdict map and the dashboard lookup both depend on (🌱 dashboard: a green PR pill should mean "the sweep would merge this now", with amber for mergeable-but-outstanding #7478).

Test-only change; no production code touched. Both functions now measure 100%; go test ./pkg/github -short passes.

Files claimed: src/pkg/github/etagcache_stats_test.go, src/pkg/github/merge_verdict_test.go (new files only) — disjoint from open PRs #7545/#7547/#7549/#7550.

Related Issue

Closes #7552

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[hivecommons-hive]

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Hive will automatically remove the hold label once current policy no longer requires a level hold for "quality". If this is an outreach PR, a human must review it and remove the label.

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[github-actions]

Changelog: this PR changes code but carries no changelog entry
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[github-actions]

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