test(engine): async behavioral tests for CrawlerEngine::crawl (#39)

[Liohtml]

Summary

Adds the missing async coverage for CrawlerEngine::crawl (#39). The crawl loop (concurrency limiting, checkpoint restore, pause, task spawning) previously had no #[tokio::test].

New tests/engine_crawl.rs drives the real loop end-to-end with no network I/O — a mock Spider in development_mode against a pre-seeded ResponseCache, so every request resolves from disk:

• crawl_processes_all_urls_from_cache_and_terminates — all seeded URLs processed via cache hit, items collected, loop terminates (a hang fails the test).
• pause_before_crawl_stops_immediately — request_pause() before crawl() breaks on the first loop check (paused == true, 0 items).
• crawl_with_single_concurrency_still_completes — concurrent_requests(1) still drains the queue and records the configured concurrency.

Test plan

• 3 new #[tokio::test]s pass; full suite + clippy + fmt green
• Hermetic — no network access (dev-cache path)

Closes #39

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Generated by Claude Code

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c4e15fb6-8d7f-4f54-b44d-a845ad162460

📥 Commits

Reviewing files that changed from the base of the PR and between df3c600 and 280c02c.

📒 Files selected for processing (1)

• tests/engine_crawl.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch claude/engine-crawl-tests

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