`base_url` with a query string produces malformed request URLs

[lesnik512]

A base_url that contains a query string is not rejected, but every relative request built from it has a broken URL. Reproduced on httpx2 2.13.1:

import httpx2

client = httpx2.Client(base_url="https://example.com/api?key=k")
print(repr(client.base_url))
# URL('https://example.com/api?key=k/')

print(client.build_request("GET", "items").url)
# https://example.com/api?key=k/items

print(client.build_request("GET", "items", params={"b": "2"}).url)
# https://example.com/api?b=2

In the first request the path ends up inside the query (key becomes k/items). In the second the path items is lost entirely. No error is raised in either case, so the request silently goes to the wrong URL.

Cause

Both steps operate on raw_path, which includes the query:

• _enforce_trailing_slash (_client.py) appends / to url.raw_path, so the slash lands after the query: /api?key=k/.
• _merge_url builds self.base_url.raw_path + merge_url.raw_path.lstrip(b"/"), so the request path is appended after the query too.

The second case then loses the path because params= replaces the whole query (#905), and the query now holds the path.

Possible fixes

1. Reject a base_url with a query string in the base_url setter, with a message pointing to params=.
2. Support it: operate on path rather than raw_path in both places and merge the base query into the request's params.

[Donimikk]

I can reproduce this on 2.13.1. There is one more case where it goes wrong: a relative URL with its own query string ends up with two ?:

client = httpx2.Client(base_url="https://example.com/api?key=k")
client.build_request("GET", "/items?x=1").url
# https://example.com/api?key=k/items?x=1

I'd like to work on a fix. Before I start, which direction do you prefer?

1. Reject a base_url with a query string, with an error that points to params=.
2. Support it: make _enforce_trailing_slash and _merge_url use path instead of raw_path, and merge the base query with the request's query.

If option 2, I'd keep the base query and the request query both, without touching how params= replaces the URL query, since that is being discussed separately in #905 / #966.

[Donimikk]

A correction to my earlier comment: switching to URL.path would decode %2F (the
regression fixed in #530, covered by test_merge_relative_url_with_encoded_slashes).
The fix should instead split raw_path at ? in _enforce_trailing_slash and
_merge_url, so the path stays encoded and the base query string is kept.