test: build mock httpware clients with transport= instead of a borrowed httpx2 client

[lesnik512]

Problem

The provider tests built an httpx2.Client by hand only to inject a MockTransport, then wrapped it with httpware.Client(httpx2_client=...). httpware does not close a client passed in that way, so _make_provider in both provider test modules returned the inner httpx2 client separately for teardown, with a comment explaining the workaround. tests/conftest.py needed two fixtures for one client.

Change

httpware 0.18.0 forwards every httpx2 client option, so the mocks pass transport= straight to httpware.Client. httpware then owns the httpx2 client and closes it along with itself.

• _make_provider (GitLab and GitHub) returns the httpware client. Call sites keep with client: unchanged, and the workaround comment is gone.
• tests/conftest.py: gitlab_client and gitlab_http merge into a single gitlab_http fixture.
• tests/integration/conftest.py: install_mock_transport builds the client in one call.
• tests/integration/README.md: describes the new construction.
• pyproject.toml: the httpware floor goes from 0.15.0 to 0.18.0, because the tests need transport= and the declared floor is what they run against.

The runtime code in semvertag/ioc.py is unchanged.

Verification

ruff, ty and the full suite pass locally (473 passed, 100% coverage) against httpware 0.18.0.