Add AsyncMegaNova client for async/await support

[flyworker]

Context

The current SDK uses synchronous requests library only (SyncTransport). Modern Python applications and agent frameworks (nova_mesh, LangChain, etc.) are async-first. An async client is needed for production integration.

Current State

• SyncTransport — synchronous HTTP via requests
• CloudTransport — synchronous for Cloud Agent API
• No async support at all

Requirements

• Add httpx as optional dependency: pip install meganova[async]
• Create AsyncTransport using httpx.AsyncClient
• Create AsyncMegaNova client mirroring all sync methods:

  from meganova import AsyncMegaNova
  
  client = AsyncMegaNova(api_key="...")
  response = await client.chat.completions.create(messages=[...])
  
  async for chunk in client.chat.completions.create(stream=True, ...):
      print(chunk)

• Create AsyncCloudAgent for async agent interaction
• Async streaming via AsyncIterator
• Add examples: examples/async_chat.py, examples/async_agent.py

Pattern

Follow OpenAI SDK pattern: openai.OpenAI() (sync) vs openai.AsyncOpenAI() (async).

[flyworker]

📋 Reference Report: AI API Compatibility Best Practices Research (29 items, 7 categories — protocols, gateways, SDKs, cloud platforms, local inference)

Tracking Epic: MeganovaAI/mega-nova-api#447

[flyworker]

Implemented in PR #15