feat: implement LiteLLM-style SmartRouter with 429 failover (Issue #286)

[teaganholland08]

This PR implements the SmartRouter logic for the Python SDK as requested in #286.

Key Features:

ArakooRouter: A flexible router that takes a list of model configurations.

429 Failover: Automatically catches Rate Limit errors and switches to the next available provider in the list.

Exponential Backoff: Implements a 2 ** attempt sleep schedule (1s, 2s, 4s...) to respect provider limits during retries.

Closes #286.

[github-actions]

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