service: guard against cross-origin __next URL redirection

[phanak-sap]

OData servers can legitimately paginate across path segments, but the
__next value is server-controlled and untrusted. Validate that its origin
(scheme + host + port) matches the configured service root before
dispatching, so session-level credentials are never forwarded to a host
the application did not configure. Raises PyODataException on mismatch.

Fix is applied in ODataHttpRequest._build_request(), the single point
both execute() (sync) and async_execute() (async) pass through, so one
check covers all dispatch paths. Depends on stdlib urllib.parse only,
keeping pyodata networking-library agnostic.

[phanak-sap]

Build will currently fail because PR #307 not yet merged.

Important should be passing of both newly added tests (passing localy)

pytest tests/test_service_v2.py::test_next_url_cross_origin_raises tests/test_service_v2.py::test_next_url_same_origin_allowed

[filak-sap]

Just do not open PRs if you know that tests are failing and another fixing the tests is pending please! Now merge the tests fixing PR and rebase this one.

[phanak-sap]

Just do not open PRs if you know that tests are failing and another fixing the tests is pending please! Now merge the tests fixing PR and rebase this one.

sure, that was indeed the intended path forward for me.