fix(types): EmbeddingResponse all-defaulted fields (v0.4.2)

[vitalii-dynamiq]

Summary

Patch release. End-to-end validation of arcllm-sdk in dynamiq's unit-test suite surfaced 10 failures from test fixtures doing `EmbeddingResponse()` with no positional args.

Root cause: arcllm 0.4.1's `EmbeddingResponse` (`msgspec.Struct`) required three positional fields (`model`, `data`, `usage`); litellm's Pydantic version is fully optional.

Fix: same pattern arcllm already uses for `ModelResponse` —

```python
class EmbeddingResponse(msgspec.Struct):
model: str = ""
data: list[EmbeddingData] = []
usage: EmbeddingUsage | None = None
object: str = "list"
```

`model_dump` emits `"usage": None` when unset to match litellm's serialised shape. Real provider responses always populate every field — this is purely caller ergonomics for fixtures and ad-hoc construction.

Validation

• `EmbeddingResponse()` no-args construction works
• arcllm's own 771 tests still pass
• mypy --strict + ruff clean
• After release: 10 dynamiq fixture-based tests previously failing should pass

Bumps to 0.4.2.

---

Note

Low Risk
Low risk patch that only relaxes EmbeddingResponse construction and tweaks its serialization; behavioral impact should be limited to callers relying on missing usage now serializing as None.

Overview
Makes EmbeddingResponse fields optional by default (matching LiteLLM fixture expectations), and updates model_dump() to emit "usage": None when usage is unset.

Bumps package version from 0.4.1 to 0.4.2 in arcllm/__init__.py and pyproject.toml.

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