Proper DeprecationWarning for 'f' cone + tighten settings validation

[bodono]

Summary

Two independent cleanups surfaced during the `scsobject.h` review:

1. `f` cone deprecation is now a real Python warning

The deprecation notice for the old `'f'` cone field was being printed via `scs_printf` to stdout. That meant users couldn't:

• Filter it with `warnings.filterwarnings`
• Capture it in logs or tests
• Promote it to an error to catch remaining usage during a migration

Switched to `PyErr_WarnEx(PyExc_DeprecationWarning, ...)`, so it integrates with the standard `warnings` machinery and `pytest.warns`. If the warning has been promoted to an error (e.g. `warnings.filterwarnings("error")`), `SCS_init` cleans up and returns `-1` with the exception propagated.

2. Tighten settings validation

Previously the Python-side checks in `SCS_init` rejected only strictly negative values for several settings. Zero would slip through Python and get rejected later by SCS's own `validate()` (in `scs_source/src/scs.c`), surfacing as a generic "ScsWork allocation error" without naming the bad setting.

Tightened to match SCS's actual rules and corrected the messages:

• `m`, `n`, `max_iters`, `acceleration_interval`, `rho_x`: `<= 0` now rejected (was `< 0`).
• `alpha`: `<= 0 || >= 2` now rejected, message "alpha must be in (0, 2)" (was `< 0` with a misleading "must be positive").
• `eps_abs`, `eps_rel`, `eps_infeas`: behavior unchanged, but message corrected from "positive" to "nonnegative" (zero is accepted).

Test plan

• `pytest test/` — 327 passed, 66 skipped (up from 322; +5 new tests)
• New tests:
  • `test_f_cone_deprecation_can_be_promoted_to_error` — verifies `filterwarnings("error")` converts the warning to a catchable exception.
  • `test_alpha_zero_raises`, `test_alpha_two_raises`, `test_rho_x_zero_raises`, `test_acceleration_interval_zero_raises` — verify each tightened bound raises `ValueError` at `init` with a specific message.
• Existing `test_deprecated_f_cone_field` and `test_f_and_z_both_set_sum` updated to assert the DeprecationWarning fires via `pytest.warns`.
• Existing `test_max_iters_zero_raises`, `test_scale_zero_raises` strengthened to match the specific error message and to trigger at `init` (no longer need the `.solve()` call that used to be where the error surfaced).