fix(types): mapping protocol on Structs (dict + **obj + iter) — v0.4.4

[vitalii-dynamiq]

Summary

Patch release. dynamiq's embedder code emits `meta = {"model": response.model, "usage": dict(response.usage)}`, which fails on a bare `msgspec.Struct` because Python falls back to integer indexing when neither the mapping protocol nor an iterable-of-pairs is recognised. arcllm 0.4.3 had `getitem/setitem` but not the full mapping protocol.

Extends `_DictLike` with:

Method	Purpose
`keys()`	Returns Struct `struct_fields` — required by `dict()`
`iter`	Yields field names (matches `dict.iter`)
`len`	Field count
`values()`, `items()`	Derived from keys
`getitem(non-str)`	Now `KeyError` instead of confusing `TypeError`

Now `dict(usage)`, `{**usage}`, `for k in usage` all behave like litellm's Pydantic Usage record.

Validation

• `dict(Usage(...))` returns a populated dict
• `for k in Struct` yields field names
• arcllm 776 → 780 unit tests (4 new `TestDictLikeAccess` regressions)
• mypy --strict + ruff clean

Bumps to 0.4.4.

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Note

Low Risk
Low risk patch: adds mapping-protocol methods to _DictLike to improve compatibility with dict(obj)/**obj and adjusts error behavior for non-string keys. Changes are localized to type ergonomics and covered by new unit tests.

Overview
Improves LiteLLM-compat dict-style behavior for msgspec.Struct response types by extending _DictLike to implement the mapping protocol (keys, __iter__, __len__, plus items/values), enabling dict(usage), **usage, and for k in usage.

Tightens __getitem__ to raise KeyError for non-string keys (avoids confusing dict() integer-index fallbacks), adds regression tests for the new behaviors, and bumps the package version to 0.4.4.

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