contexts_by_config accumulated browser contexts unboundedly in long-running
crawlers (Docker API). Two root causes fixed:
1. _make_config_signature() hashed ~60 CrawlerRunConfig fields but only 7
affect the browser context (proxy_config, locale, timezone_id, geolocation,
override_navigator, simulate_user, magic). Switched from blacklist to
whitelist — non-context fields like word_count_threshold, css_selector,
screenshot, verbose no longer cause unnecessary context creation.
2. No eviction mechanism existed between close() calls. Added refcount
tracking (_context_refcounts, incremented under _contexts_lock in
get_page, decremented in release_page_with_context) and LRU eviction
(_evict_lru_context_locked) that caps contexts at _max_contexts=20,
evicting only idle contexts (refcount==0) oldest-first.
Also fixed: storage_state path leaked a temporary context every request
(now explicitly closed after clone_runtime_state).
Closes#943. Credit to @Martichou for the investigation in #1640.
c790231aba