[19.0][FIX] test-requirements: use httpx2 for starlette TestClient

[agiuseppe28]

On 19.0, test-requirements.txt pins httpx, but starlette's TestClient in this stack imports httpx2 (import httpx2 as httpx). Without httpx2 installed, the TestClient falls back to httpx and emits a StarletteDeprecationWarning, which the OCA CI log check treats as an error — failing test with Odoo / test with OCB for fastapi and every module depending on it (e.g. #619).

The 18.0 branch already uses httpx2 here; the change didn't carry over when 19.0 diverged. This restores it, unblocking the test CI for fastapi-dependent modules on 19.0.

[agiuseppe28]

@glitchov could you please approve? This would unlock several migrations. Thanks!

[rvalyi]

indeed, already httpx2 in 18.0 but as the files is not part of any Odoo module it was not carried to the 19.0 branch https://github.com/OCA/rest-framework/blob/18.0/test-requirements.txt#L2

[rvalyi]

trivial, fast tracking...