fix(pydantic): propagate SDK internal exceptions

[1fanwang]

The pydantic model wrapper currently turns Restate SDK control-flow exceptions into a plain Exception. That makes suspension and retry control paths look like ordinary model-call failures, so callers have to inspect __cause__ to recover the real Restate exception.

This keeps SdkInternalBaseException and its subclasses unchanged when they leave the journaled pydantic model call. The same change applies to streaming model calls. I left provider 4xx classification out because the issue proposes a policy, but there is no maintainer-confirmed mapping yet.

Part of #225

Testing Done

Raw logs

Red, with the old wrapper implementation restored over the new regression test:

$ git checkout HEAD^ -- python/restate/ext/pydantic/_model.py
$ uv run -m pytest tests/ext_pydantic_model.py -q
...
E           Exception: Internal error during model call
python/restate/ext/pydantic/_model.py:85: Exception
...
E           Exception: Internal error during model stream call
python/restate/ext/pydantic/_model.py:125: Exception
=========================== short test summary info ============================
FAILED tests/ext_pydantic_model.py::test_request_propagates_sdk_internal_exception
FAILED tests/ext_pydantic_model.py::test_request_stream_propagates_sdk_internal_exception
2 failed in 0.83s

Green, after restoring this branch's wrapper implementation:

$ git checkout HEAD -- python/restate/ext/pydantic/_model.py
$ uv run -m pytest tests/ext_pydantic_model.py -q
..                                                                       [100%]
2 passed in 0.61s

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}