Fix: Preserve custom CookiePolicy in Session.prepare_request

[Ajay9704]

Summary

This PR fixes an issue where custom cookie policies set on a Session's cookie jar were not being preserved during request preparation.

Details

In Session.prepare_request(), when merging cookies from the session and request, the code was creating a new RequestsCookieJar() without preserving the cookie policy from the session's cookie jar. This meant that any custom cookie policy (like MyCustomCookiePolicy in issue #7122) would be lost during request preparation.

Fix

Modified the cookie merging logic to check if the session's cookies are a RequestsCookieJar and, if so, preserve the cookie policy by copying it to the new jar using set_policy().

Testing

The fix has been tested to ensure:

• Custom cookie policies are now preserved during Session.prepare_request
• Policy methods are properly consulted during request preparation
• Restrictive policies correctly prevent cookies from being sent
• Permissive policies still allow cookies to be sent as expected
• Existing functionality remains unaffected

Fixes #7122