fix: avoid duplicate final crawler persistence

[AkhilTrivediX]

Summary

Avoid duplicate final persistence during BasicCrawler.run() shutdown.

• Stop statistics before crawler teardown so the final statistics record is written once with its completion timestamp.
• Let a crawler-owned event manager emit the final persistence event after the session pool listener is removed.
• Keep the explicit persistence event for shared event managers, while preventing SessionPool.teardown() from writing the same state a second time.

Root cause

The shutdown path emitted PERSIST_STATE, then directly persisted the session pool and statistics again. A crawler-owned event manager emitted another final event as it closed, which added a second write for statistics.

Validation

• corepack yarn vitest run test/core/crawlers/basic_crawler.test.ts --silent (70 passed)
• corepack yarn eslint packages/core/src/session_pool/session_pool.ts packages/basic-crawler/src/internals/basic-crawler.ts test/core/crawlers/basic_crawler.test.ts
• corepack yarn tsc-check-tests
• corepack yarn biome format --write ...
• git diff --check

Fixes #3167