test: add two more tests exercising the adapter

[allrob23]

This PR adds two test cases for Session.mount() to cover the documentation's recommendation about trailing slashes in prefixes (see #6935).

The first test test_session_get_adapter_prefix_with_trailing_slash verifies that prefixes with a trailing / (e.g., https://example.com/) match only the intended hostname.

The second test test_session_get_adapter_prefix_without_trailing_slash verifies that prefixes without a trailing / (e.g., https://example.com) match both the intended hostname and extended hostnames (e.g., https://example.com.other.com), as warned in the docs.

Together, these tests ensure the longest prefix match behavior is well-documented and stable.

[allrob23]

I don't think the error in ci is related to my modification, could someone help?

[allrob23]

about the lint, I opened another PR #6937 to solve it

[sigmavirus24]

Please rebase this on the latest HEAD of main. This will pull in your fix for the lint action

[allrob23]

@sigmavirus24 is it fine? I don't know about the ci fail in 3.9 windows

[sigmavirus24]

@allrob23 yes, we dropped pypy 3.9 so this looks good to me