fix: handle Windows IM attachment paths

[Gucc111]

Summary

• Replace slash-based IM attachment path containment checks with a cross-platform path.relative guard.
• Fix Windows runs incorrectly rejecting downloaded IM attachments as escaping their target directory.

Test plan

• npm run build
• node --test --test-force-exit --test-timeout 60000 "dist/tests/adapters/im-attachment-store.spec.js" (run before dropping the added test file per follow-up request)
• node --test --test-force-exit --test-timeout 60000 "dist/tests/gateway/attachment-guidance.spec.js"
• node --test --test-force-exit --test-timeout 60000 "dist/tests/adapters/feishu-permission-reply.spec.js"

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