fix: preserve custom CookiePolicy in Session.prepare_request

[shindonghwi]

Summary

When using a custom CookiePolicy on a session's CookieJar, the policy was being discarded when Session.prepare_request() was called. This happened because the code was creating a new RequestsCookieJar() with the default policy instead of copying the session's jar with its policy preserved.

This fix uses the existing _copy_cookie_jar() function to properly preserve the CookiePolicy when merging cookies.

Related Issues

• Fixes Unable to override cookie policy in Session.prepare_request #7122
• Related to Unable to override cookie policy in Session.prepare_request #3416 (originally reported)
• Related to PR Persist Session-level CookiePolicy #4042 (original fix that was merged to the abandoned proposed/3.0.0 branch in 2017)

Changes

• Import _copy_cookie_jar in sessions.py
• Use _copy_cookie_jar(self.cookies) instead of merge_cookies(RequestsCookieJar(), self.cookies) in prepare_request()
• Added test case test_cookie_policy_preserved_on_prepare_request

Test Plan

from requests.cookies import RequestsCookieJar
from http.cookiejar import DefaultCookiePolicy
import requests

class BlockAllCookies(DefaultCookiePolicy):
    def set_ok(self, cookie, request):
        return False

s = requests.Session()
jar = RequestsCookieJar(policy=BlockAllCookies())
s.cookies = jar

req = requests.Request('GET', 'https://example.com/')
prepared = s.prepare_request(req)

# Before fix: DefaultCookiePolicy
# After fix: BlockAllCookies
print(type(prepared._cookies.get_policy()))

[nateprewitt]

Thanks for the PR, @shindonghwi. I think we'll move #4042 onto main if we decide to move forward with this.