v0.9.0 — native async transport

Added

• Native async transport behind a new [async] extra
  (pip install saia-python[async], pulls httpx). New saia_python.aio
  module: AsyncSAIAClient + AsyncChatService / AsyncArcanaService /
  AsyncModelsService, the httpx.AsyncClient twins of the sync data plane —
  chat completions and ARCANA RAG chat (streaming + non-streaming),
  get_rate_limits, and health_check. Use it as an async context manager;
  from saia_python import AsyncSAIAClient resolves lazily so importing the
  package never pulls httpx for sync-only users. See ADR-0007.
  • The retry brains are shared, not copied. aexecute /
    apost_chat_completion reuse RetryPolicy / _plan / _jitter /
    resolve_retry / parse_rate_limits verbatim, so the async path honours the
    same 429 policy (ADR-0006) as the sync path and the two cannot drift. The
    retry keyword works identically (retry=False fails fast; a RetryPolicy
    tunes it).
  • AsyncSSEStream owns the httpx client.stream(...) context, retrying
    an initial 429 before the body is exposed (never mid-stream). Two
    consumption modes: async for chunk in stream (decoded dicts; raises on an
    error status) or async for line in stream.aiter_lines() (raw lines, no
    raise — lets a gateway frame upstream errors itself).
• Informative 429 errors (format_rate_limit_error, exported). When retry is
  off or the budget is spent, RateLimitError now carries a human-readable
  message — which window was hit, when it resets, and how to auto-retry — instead
  of only the bare server string. Shared by both transports (raised from
  raise_for_status).
• Pure request builders (saia_python._payloads, re-exported at top level):
  build_chat_body, apply_arcana_fields, arcana_chat_headers, and the
  INFERENCE_SERVICE constant. Transport-free, so the sync services, the async
  services, and external gateways (the AVOR adapter) share one definition of
  the three-part ARCANA injection invariant (enable-tools + arcana.id body
  fields + the inference-service header) — defined and unit-tested once.

Changed

• Every SAIAError now carries status_code + response_body (previously
  only APIError did). Lets a caller reframe the exact upstream response.
  RateLimitError keeps its rate_limits attribute (unchanged signature).
• raise_for_status is transport-agnostic — it accepts a requests.Response
  (honouring .ok) or an httpx.Response (deciding success from status_code),
  so one implementation serves both paths. No behaviour change for sync callers.
• The test extra now installs [async] too, so CI exercises the async suite.

Notes

• Async covers the data plane + read-only control-plane calls. File
  upload/index/sync, voice, and document conversion stay sync-only on
  SAIAClient (batch/admin work, blocking file I/O, no concurrency benefit). See
  ADR-0007 for the scope rationale.
• Tests: +42 async tests (test_async_transport / _streaming / _arcana /
  _chat / _client, test_payloads, test_rate_limit_message); the suite
  grows 198 → 240 and stays mypy- and ruff-clean.