Stop _gl_context from converting test failures into skips

[LeSingh1]

_gl_context() yields from inside a try whose handler turns everything into a skip:

    try:
        ...create context and texture...
        yield int(tex_id.value), int(target)

    except Exception as e:
        # Convert any pyglet/GL creation failure into a clean skip
        pytest.skip(f"Could not create GL context/texture: {type(e).__name__}: {e}")

@contextmanager re-raises the with-body's exception at the yield point, so
except Exception also catches failures from the test body. AssertionError is an
Exception; pytest's Skipped is a BaseException, so the replacement skip escapes
cleanly and the run is reported as SKIPPED.

That means neither assertion in test_cuda_gl_register_image_smoketest can fail the suite:

        assert name in acceptable, f"cudaGraphicsGLRegisterImage returned {name}"
        if name == "cudaSuccess":
            assert int(resource) != 0

A genuinely wrong cudaGraphicsGLRegisterImage return is reported as
Could not create GL context/texture: AssertionError: .... The handler's own comment says
"creation failure", which is what it was meant to cover.

Demonstrated with a minimal standalone reproduction of the same shape:

test_yield_in_try.py::test_body_failure_under_main_shape   SKIPPED
    Could not create GL context/texture: AssertionError: this assertion should FAIL the test
test_yield_in_try.py::test_body_failure_under_fixed_shape  FAILED
    AssertionError: this assertion should FAIL the test

Both tests in this module are on the always-skipped list in #2077, so this has never been
visible in CI.

Fix

Move the context/texture creation into _create_gl_texture() so the handler wraps only
that, and keep the yield outside it. Cleanup stays in the outer finally, so a
partially built context is still torn down on the skip path. tex_id is now initialized,
which also removes an UnboundLocalError that the finally block was silently swallowing
when creation failed before the texture existed.

pytest.importorskip and the no-EGL pytest.skip inside _create_gl_texture still
propagate correctly — Skipped is a BaseException, so except Exception does not catch
them.

Test

test_gl_context_lets_body_failures_fail stubs _create_gl_texture() and asserts that an
AssertionError raised inside the with body propagates. No GL context, no display, no
GPU, no pyglet — so unlike the two smoketests it actually runs in CI. I verified it
passes against the restructured contextmanager and that the pre-existing shape converts
the same failure into a skip.

ruff check and ruff format --check are clean.

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