Remove _merge_url_query once httpx2 merges params into the URL query

[lesnik512]

Context

httpx2 replaces a URL's query string whenever params are sent, including client-level params (pydantic/httpx2#905). httpware works around this in src/httpware/client.py: #145 (0.17.1) added the workaround, and #156 replaced it with _build_request, which strips the URL's query, lets httpx2 build the request, then puts the original query bytes back in front of the params. stream() goes through _stream / _stream_async for the same reason. Semantics match requests and pydantic/httpx2#966: params are appended after the URL's query, duplicate keys are kept, and the URL's query bytes are not re-encoded.

Blocked on

An httpx2 release that merges params into the existing URL query. Candidate fix: pydantic/httpx2#966 (open, unreviewed as of 2026-10-03). An earlier attempt, pydantic/httpx2#1026, was closed unmerged.

When unblocked

1. Check upstream semantics still match ours: params appended after the URL query, duplicate keys kept, client params before per-request params, params={} keeps the URL query, URL query bytes unchanged.
2. Raise the httpx2 floor in pyproject.toml to the release containing the fix.
3. Replace every _build_request(client, method, url, kwargs) call (build_request, _prepare_request and the capped stream helpers, in both worlds) with client.build_request(method, url, **kwargs), and delete _build_request. The uncapped stream() path can go back to self._httpx2_client.stream, dropping _stream / _stream_async.
4. Keep tests/test_url_query_merge.py: it passes 50/50 against #966 (2631c38) with the workaround stubbed out, so it should pass unchanged. If a case fails, upstream semantics differ from #966 and that is a behaviour change to call out in the release.

[lesnik512]

Ran tests/test_url_query_merge.py against pydantic/httpx2#966 (2631c38) with _merge_url_query stubbed to a pass-through. #966 appends params to the existing query instead of merging, so 3 of 7 cases fail:

Case	Ours	#966	Kind
params-override-same-key (?a=1&b=1 + b=2)	a=1&b=2	a=1&b=1&b=2	Different meaning
url-query-overrides-client (?a=1&c=1, client c=3, b=2)	c=1&a=1&b=2	a=1&c=1&c=3&b=2	Different meaning
client-params-keep-url-query	c=3&a=1	a=1&c=3	Order only

The other four cases pass. With the workaround left in place, the whole suite passes on #966.

If #966 lands as written, removing _merge_url_query would mean per-request params no longer override a key already in the URL, and that key would be sent twice.

[lesnik512]

Since the analysis above: requests also appends (?a=1 + params={"a": "2"} gives ?a=1&a=2), so #966's behaviour matches requests and httpware's override was the outlier. #156 switches httpware to append and keeps the URL's query bytes unchanged. With it, tests/test_url_query_merge.py passes 50/50 against #966 with the workaround stubbed out. Once #156 merges, step 1 here holds as-is, and step 3 means deleting _build_request and the _stream/_stream_async helpers rather than _merge_url_query.