feat: support "auth-int" quality-of-protection when using DigestAuth

[dsillman2000]

Summary

Adding support for "auth-int" (message integrity authentication) quality-of-protection implementation, as specified in RFC-7616.. Additionally, updates unit tests to reflect support for the "auth-int" QoP.

Note on "example" scenario from RFC-7616

I had to change the expected response hashes from the official ones stated in the RFC-7616 Section 3.9.1 example since they don't provide an example where the client chooses to use "auth-int" QoP.

That being said, I was able to check the correctness of my implementation against httpbin using the /digest-auth/auth-int/Mufasa/CircleofLife/[MD5|SHA-256] endpoints to verify that a proper 200 status code is returned, meaning the httpbin server implementation of "auth-int" authentication agrees with the implementation added to this feature branch. Httpbin appears to support "http" requests to this endpoint, but doesn't properly support "https" requests (unexpected EoF encountered).

Integration test with HTTPBin

from httpx import Client, DigestAuth, Request

if __name__ == "__main__":
    auth = DigestAuth(username="Mufasa", password="CircleofLife")
    # Note that I'm using locally-hosted HTTPBin over Docker since the official website is down today, Dec 14 2025.
    request = Request(
        "GET",
        # "http://httpbin.org/digest-auth/auth-int/Mufasa/CircleofLife/MD5",
        # "http://httpbin.org/digest-auth/auth-int/Mufasa/CircleofLife/SHA-256",
        # "http://0.0.0.0:80/digest-auth/auth-int/Mufasa/CircleofLife/MD5",
        "http://0.0.0.0:80/digest-auth/auth-int/Mufasa/CircleofLife/SHA-256",
    )

    with Client(auth=auth) as client:
        response = client.send(request)
        print(response.headers.raw)
        print(response.content)
        print(response.status_code) # We get a HTTP code 200 "OK"

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

GH Actions test failures

GH Actions seem to be failing non-deterministically for the test_proxies.py module (no changes from this PR), particularly due to timeouts when talking to example.com. I encourage a project maintainer to try re-running the checks for this PR during their review to demonstrate the non-deterministic behavior (take note of which python versions fail each time, if any).

Is there a default timeout being used for the tests which we can make more permissive?