Preserve leading slashes in request path_url

[typhon8]

Summary

Fixes issue #6711 where S3 presigned URLs with keys starting with '/' were incorrectly modified, breaking URL signatures.
URLs like https://bucket.s3.amazonaws.com//key_name now correctly preserve //key_name in the path.

Changes

• Remove URL manipulation that collapsed leading slashes in request_url() method
• Add test to verify leading slashes are preserved for S3-style URLs

Reproduction

import requests

# S3 presigned URL with key starting with '/'
url = "https://bucket.s3.amazonaws.com//key_with_leading_slash.txt"
r = requests.get(url)  # Previously broken, now works correctly
Fixes #6711

[nateprewitt]

Hi @typhon8, thanks for the PR. Unfortunately, this is breaking. We specifically changed the behavior to handle ambiguous paths failing in urllib3.

I've added urllib3/urllib3#4952 to urllib3 which should fix the behavior in their next release. At that point we can revisit removing this from Requests. We do not want to remove the test though, that is required to still function before we make any changes.

[nateprewitt]

I made the test tweak to go back to the old format. It demonstrates the same thing without adding s3 semantics into the test suite. Once the tests finish, we can merge this and it will go out with 2.34.0.