fix(decoders): MsgspecDecoder.can_decode rejects nested CustomType (0.9.1)

[lesnik512]

Summary

Two linked deliverables from a post-0.9.0 review pass:

1. Delta audit of the 0.9.0 multi-decoder epic — a verified, severity-bucketed audit (planning/audit/2026-06-12-delta-audit.md) of everything changed since 0.8.6, run via a single-chunk find→verify→synthesize workflow. 14 confirmed findings (2 high, 1 medium, 3 low, 8 nit).
2. The fix for the headline High finding — MsgspecDecoder.can_decode no longer falsely claims parameterized containers of pydantic models.

The bug

can_decode inspected only the top-level msgspec.inspect.type_info node. For list[PUser] (a ListType whose item_type is a CustomType), the guard passed, msgspec built a decoder via its CustomType fallback, and can_decode returned True. The MissingDecoderError pre-flight was bypassed, a real request was sent, and decode then failed with DecodeError — cached per-instance, so every later request of that shape repeated the wasted round-trip. Masked under the pydantic-first default; only bit msgspec-only configs.

The fix

A recursive _contains_custom_type walker rejects when a CustomType appears anywhere in the type tree — covering list/dict/set/tuple/Optional/Union and arbitrary nesting via generic introspection. It stops at Struct/dataclass field boundaries automatically (those expose Field, not Type), so valid targets like list[SomeStruct] stay accepted and self-referential structs can't loop. No public-API or ResponseDecoder protocol change.

Ships as 0.9.1 (release notes drafted in planning/releases/0.9.1.md; tagging is a separate manual step).

Test Plan

• just lint clean (ruff format + check + ty)
• just test — 501 passed, 100% coverage
• Unit: parametrized rejection (list[PUser], dict[str, PUser], dict[PUser, str], PUser | None, tuple[PUser, int], list[list[PUser]], set[PUser]) + acceptance (list[Struct], dict[str, int], scalars) sets
• Integration: sync + async msgspec-only client raises MissingDecoderError for list[PUser] with the transport handler asserted never invoked
• Regression proven genuine: reverting only the source fix makes both dispatch tests fail (request sent)

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