[Migration] Drop the constant parameters and *_Intercept_centered RVs bambi 0.20 keeps in posterior

Part of #1305. Parent: #1306.

⚠️ **Unmasked by #1313** — these ids were hidden under the R5 mark until `sample()` worked again.

bambi 0.20 records two kinds of variables in `posterior` that the old bambi never stored:

- every constant marginal parameter is built as a `pm.Deterministic` and traced — `p_outlier=0.05`, a fixed `z=0.5`, and the `0.0` placeholder HSSM uses for a fixed-vector parameter;
- the `*_Intercept_centered` RV is kept alongside the uncentered `*_Intercept` deterministic ([bambi#1002](https://github.com/bambinos/bambi/pull/1002), *Predictor centering and the intercept*: "The intercept on the original scale is now a deterministic variable in the model").

```
assert "v" not in idata.posterior.data_vars # fixed-vector v recorded as 0.0
assert summary.shape[0] == 4 # 5: p_outlier is a row now
assert summary.shape[0] == 6 # 8: + p_outlier, v_Intercept_centered
```

So `az.summary` / `plot_trace` gain rows and fixed-vector parameters show up in the trace as a constant.

### Tasks
- [ ] Decide whether `_clean_posterior_group` (`src/hssm/base.py:900`) should drop constant deterministics and `*_centered` RVs, or whether this is an `omit_offsets`-style opt-in
- [ ] Coordinate with #1314, which touches the same function (`distributional_components` → `conditional_parameters`)
- [ ] Remove the R13 xfail marks

### Impact
5 test ids across 2 files: `tests/test_hssm.py::TestFixedVectorParams` (2), `tests/integration/test_mcmc.py::test_post_processing_*` (3).

Details: `design/bambi-dev-upgrade-test-failures.md` (R13), `design/bambi-migration-fix-plan.md` (F12).

Start with _clean_posterior_group in src/hssm/base.py:900 and review design/bambi-dev-upgrade-test-failures.md (R13) and design/bambi-migration-fix-plan.md (F12). Coordinate the decision with #1314, then run the five affected tests in tests/test_hssm.py::TestFixedVectorParams and tests/integration/test_mcmc.py::test_post_processing_*; done means the expected posterior shapes and variables pass without the R13 xfails.

Written by the indexing model from the issue text.