feat!: multi-decoder routing (decoders=[...])

[lesnik512]

Summary

Replaces the single-decoder slot on AsyncClient/Client with a type-dispatched decoders=[...] list. Reverses the 0.3.0 fail-fast for missing pydantic — AsyncClient() no longer raises on missing extras; failure is deferred to the first response_model= use site via the new MissingDecoderError (fires before the HTTP call).

Headline changes

• ResponseDecoder Protocol gains can_decode(model) -> bool. The client walks _decoders in order and picks the first whose predicate returns True. Built-in decoders claim broadly within their library; native types of the other library are rejected (pydantic rejects msgspec.Struct; msgspec rejects pydantic.BaseModel via msgspec.inspect.type_info + CustomType filter).
• decoders: Sequence[ResponseDecoder] | None = None replaces the old decoder= kwarg on both AsyncClient.__init__ and Client.__init__. Default None resolves via _build_default_decoders() against installed extras (pydantic-first when both present, empty tuple when neither).
• MissingDecoderError (sibling of DecodeError under ClientError) carries model: type and registered_names: tuple[str, ...]. Pre-flight check at .send() / .send_with_response() entry — distinct from DecodeError (decoder ran, payload bad).
• Mixed pydantic + msgspec in one client. AsyncClient(decoders=[PydanticDecoder(), MsgspecDecoder()]) is the new default; BaseModel routes to pydantic, Struct to msgspec, shared shapes (dict, list, dataclass, primitive) go to the first decoder in the list.
• Sync/async surfaces are true mirrors — both classes refactored atomically; legacy _default_pydantic_decoder() helper deleted.

Behavioral changes for users

• AsyncClient(decoder=...) now raises TypeError: unexpected keyword argument 'decoder'. Migration: AsyncClient(decoders=[...]).
• AsyncClient() / Client() no longer raise ImportError when pydantic is missing. Errors now surface only when response_model= is used.
• Custom ResponseDecoder implementations must add can_decode. Trivial catch-all migration: def can_decode(self, model): return True.

Spec + plan

• Spec: planning/specs/2026-06-09-multi-decoder-design.md
• Plan: planning/plans/2026-06-09-multi-decoder-plan.md
• Seam B updated in planning/engineering.md.

Target release

0.9.0 — minor bump, breaking surface (! in commit subjects).

Test plan

• just lint clean (ruff format + ruff check + ty)
• just test green — 485 passed, 100% coverage
• mkdocs build --strict clean (no warnings on new content)
• Sync/async symmetry verified by code reviewer (AsyncClient and Client read identically modulo await)
• Pre-flight contract verified — MissingDecoderError fires before _dispatch(request) (asserted via pytest.fail in MockTransport handlers in 4+ tests)
• Cross-task coherence reviewed — no residue of old decoder= kwarg anywhere in src/ / tests/ / docs/
• Manual smoke test: pip install httpware[pydantic,msgspec] and verify mixed-decoder client works end-to-end
• Manual smoke test: pip uninstall pydantic, verify AsyncClient() constructs without raising