fix(types): _DictLike mixin for response Structs (v0.4.3)

[vitalii-dynamiq]

Summary

Patch release. End-to-end validation in dynamiq's unit-test suite surfaced the litellm-fixture-compat gap: tests assemble response objects with dict-style assignment (`response["data"] = [...]`), a pattern that worked on litellm's Pydantic-derived models but not on arcllm's strongly-typed `msgspec.Struct` records.

Adds a `_DictLike` mixin defined once and applied to all 17 public Struct types in `arcllm.types`:

Pattern	Resolves to
`response[key]`	`getattr(response, key)` (raises KeyError if absent)
`response[key] = value`	`setattr(...)` — no type validation (litellm-style loose typing escape hatch)
`key in response`	field exists and is non-None
`response.get(key, default)`	safe attribute lookup with default

Real provider code keeps the canonical attribute path with full msgspec type-checking. The loose surface is for tests and ad-hoc mutation.

Validation

• arcllm own suite — 776 pass (5 new `TestDictLikeAccess` regression tests)
• mypy --strict + ruff + format — clean
• After release: 10 dynamiq embedder fixture tests should pass (was failing on `EmbeddingResponse["data"] = ...`)

Bumps to 0.4.3.

---

Note

Medium Risk
Moderate risk because it changes behavior of all public msgspec.Struct response types by allowing post-construction mutation via __setitem__ without validation, which could mask type issues or alter downstream expectations.

Overview
Adds a _DictLike mixin to make all public response msgspec.Struct types support LiteLLM-style dict semantics (obj[key], obj[key]=..., key in obj, obj.get(...)) while preserving existing attribute access.

Bumps package version to 0.4.3 and adds regression tests ensuring item access works, unknown keys raise KeyError, and __contains__ reflects non-None fields.

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Cursor Bugbot has reviewed your changes and found 2 potential issues.

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get returns None when __contains__ reports key absent

Medium Severity

The get method and __contains__ have inconsistent semantics for None-valued struct fields. __contains__ treats a field with value None as absent (returns False), but get uses bare getattr which finds the attribute and returns None rather than the caller's default. This breaks the standard dict contract: if "usage" not in response is True, callers expect response.get("usage", fallback) to return fallback, but it returns None instead.

Additional Locations (1)

• arcllm/types.py#L52-L54

 

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