Prevent scrapy shell from starting spider requests (#7557)

* Prevent scrapy shell from starting spider requests

* Run pre-commit fixes

* Update shell architecture notes

---------

Co-authored-by: Mayuresh <mayuresh@Mayureshs-MacBook-Air.local>

scrapy/shell.py

@@ -28,11 +28,7 @@ from scrapy.spiders import Spider
 from scrapy.utils.conf import get_config
 from scrapy.utils.console import DEFAULT_PYTHON_SHELLS, start_python_console
 from scrapy.utils.datatypes import SequenceExclude
-from scrapy.utils.defer import (
-    _schedule_coro,
-    deferred_f_from_coro_f,
-    maybe_deferred_to_future,
-)
+from scrapy.utils.defer import deferred_f_from_coro_f, maybe_deferred_to_future
 from scrapy.utils.misc import load_object
 from scrapy.utils.reactor import is_asyncio_reactor_installed, set_asyncio_event_loop
 from scrapy.utils.response import open_in_browser
@@ -69,8 +65,8 @@ if TYPE_CHECKING:
 # 3. When fetch() is called, it prepares a request and calls Shell._schedule()
 # in the reactor thread (via threads.blockingCallFromThread()).
 # 4. Shell._schedule() calls Shell._open_spider() (on the first call).
-# 5. Shell._open_spider() calls engine.open_spider_async(close_if_idle=False)
-# and engine._start_request_processing().
+# 5. Shell._open_spider() calls
+# engine.open_spider_async(close_if_idle=False).
 # 6. Shell._schedule() calls engine.crawl(request), scheduling the request.
 # 7. Shell._schedule() via _request_deferred() waits until the request callback
 # is called. When it's called, the response becomes available.
@@ -214,7 +210,6 @@ class Shell:
         self.crawler.spider = spider
         assert self.crawler.engine
         await self.crawler.engine.open_spider_async(close_if_idle=False)
-        _schedule_coro(self.crawler.engine._start_request_processing())
         self.spider = spider
 
     def fetch(