tests: extend cmd/cosift coverage round 2

[TeoSlayer]

Summary

• Covers recallAtK and summary from bench_pq.go, including empty/single/many-element edge cases.
• Covers truncStr and truncateForEmbedLite boundary behaviour.
• Covers pebble_serve middleware (stripPort, numNonEmpty, newRateLimiterFromEnv, rateLimiter.allow/whitelistList, rateLimit, count) and the no-receiver/whitelist/exhaust branches.
• Covers static asset handlers: handleLanding (exact-root + 404), handleChat, handleOpenAPI, handleSwaggerUI, handleSwaggerAsset (CSS/JS/404), handleHealthz, responseRecorder.
• Covers forwardURLToPeer (happy, 5xx error, full-URL prefix branches) via httptest.NewServer.
• Covers batchEmbed / embedWithRetry happy + multi-batch + non-rate-limit error paths using a fake embed.Embedder.
• Covers paraphraseRetriever.generateParaphrases cache hit + chat error path using a fake embed.ChatClient.
• Covers auth-gated admin handlers' unauthorized branch: handleCheckpoint, handlePQEncode (+ no-HNSW 400), handlePQTrain, handleEmbedBackfill (+ missing-deps 501), handleHNSWCompact, handleFrontierPurgeHost, handleSitemapImport, handleSitePack, handleCrawlEnqueue, handleCrawlNow, handleEvalQuick, handleWETImport, handleRSSImport.
• Covers runStats (sqlite happy + pebble empty + unknown-backend error), runGC (with/without paraphrase TTL), runOutcomes (JSON, CSV, bad-format), runStatusFile (missing, bad-JSON, human, JSON, target-reached, stale-warning), runDoctor (local-only human + JSON).

go test -short -race ./cmd/cosift/ rises from 25.6% → 32.1% (+6.5pp).

Test plan

• go test -short -race ./cmd/cosift/ passes locally
• coverage delta confirmed via go test -coverprofile
• CI green

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!