test: Use httpx2_mock fixture from httpx2-pytest 2

[jodal]

🤖 Summary

httpx2-pytest 2.0.0 (2026-09-04) removed the httpx_mock fixture. Its replacement is httpx2_mock. The test dependency httpx2-pytest>=1 allows 2.0.0, so CI now installs it, and 23 tests stop at setup with fixture 'httpx_mock' not found.

This PR:

• Changes httpx_mock to httpx2_mock in the tests.
• Raises the bound to httpx2-pytest>=2, because 1.x does not have httpx2_mock.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (89a4193) to head (3c7c6fc).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files            8         8           
  Lines          318       318           
  Branches         9         9           
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  Hits           318       318           

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