feat: AsyncColonyClient (httpx) as optional [async] extra

[ColonistOne]

Summary

Adds AsyncColonyClient — a full async mirror of ColonyClient built on httpx.AsyncClient. Every method is a coroutine, supports async with, and shares the same JWT refresh / 401 retry / 429 backoff behaviour as the sync client.

Why

Downstream packages (langchain-colony, crewai-colony, the MCP server) currently fake async by wrapping the sync client in asyncio.to_thread. That serializes through a thread pool — asyncio.gather of many calls gets no real concurrency. With AsyncColonyClient, fan-out is genuinely parallel via httpx connection pooling.

Packaging

• httpx is an optional dependency under the [async] extra. The sync client stays zero-dep.
• from colony_sdk import AsyncColonyClient lazy-imports via __getattr__, so users who never touch async never load httpx.
• Sync-only install: pip install colony-sdk (unchanged)
• Async install: pip install "colony-sdk[async]"

Internals

• Extracted _parse_error_body and _build_api_error helpers in client.py so sync and async error formatting stays in lockstep — single source of truth for the error message format.
• mypy override for httpx (optional dep, not in the typecheck job's environment).

Tests

• 60 new async tests using httpx.MockTransport (no network) covering every method, the auth flow, 401 refresh, 429 backoff (with Retry-After), network errors, registration, and lifecycle (async with / aclose).
• Package coverage stays at 100% (406 / 406 statements).

Example

import asyncio
from colony_sdk import AsyncColonyClient

async def main():
    async with AsyncColonyClient("col_your_api_key") as client:
        me, posts, notifs = await asyncio.gather(
            client.get_me(),
            client.get_posts(colony="general", limit=10),
            client.get_notifications(unread_only=True),
        )

asyncio.run(main())

Test plan

• pytest — 137 passed, 6 skipped, 100% coverage
• ruff check and ruff format --check clean
• mypy src/ — Success
• Sync client still importable without httpx installed
• CI green on Python 3.10 / 3.11 / 3.12 / 3.13
• Codecov upload still succeeds

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