fix!: append params after the URL's own query and keep its bytes

[lesnik512]

Changes how a URL's own query string combines with params, to match requests and the pending upstream fix (pydantic/httpx2#966). It also stops httpware from re-encoding that query string.

Behaviour change

params (client-level, then per-request) are now appended after the URL's query. Duplicate keys are kept instead of overridden.

URL	params	client params	Before	After
?a=1&b=1	b=2	-	a=1&b=2	a=1&b=1&b=2
?a=1&c=1	b=2	c=3	c=1&a=1&b=2	a=1&c=1&c=3&b=2
?a=1	-	c=3	c=3&a=1	a=1&c=3

Code that put a key in the URL and relied on params to replace it will now send the key twice. Release notes should call this out.

Bug fixed: the URL's query was re-encoded

Before, when any params applied, the URL's query was parsed and rebuilt: ?q=a%20b went out as q=a+b and ?flag as flag=. That can break signed URLs and servers that are strict about encoding. Now the original bytes are sent unchanged (new test case url-query-bytes-kept).

How

httpx2 sends params through str(QueryParams(...)), which re-encodes them, so nothing passed as params keeps the URL's original bytes. _build_request replaces _merge_url_query. It strips the URL's query, lets httpx2 build the request, then puts the original query bytes back in front of whatever query httpx2 built.

To route the uncapped stream() path through _build_request, it now uses _stream / _stream_async. These mirror httpx2.Client.stream: build_request, then send(stream=True), then close on exit. The capped path already worked this way. The request is still built lazily, inside the exception mapper.

Relation to #146

tests/test_url_query_merge.py now matches upstream behaviour. Against pydantic/httpx2#966 (2631c38) with _build_request stubbed to a pass-through, all 50 tests pass. Against httpx2 2.13.1 with the same stub, 43 fail, so the workaround is still needed until a release includes the fix. Once one does, #146 is a plain deletion with no behaviour change.

Verification

just lint-ci passes, and just test-ci passes 1007 tests at 100% coverage.