docs(spec): add multi-decoder routing design

Reframes the decoder seam from one-decoder-per-client to a typed-dispatched
`decoders=[...]` list. Removes the 0.3.0 eager-import fail-fast: `AsyncClient()`
no longer raises on missing pydantic. Adds `MissingDecoderError` (fires before
the HTTP call). Target release: 0.9.0.

Author: lesnik512