Fix FallbackModel behavior for AWS Bedrock throttling

[cjermain]

This PR fixes FallbackModel behavior for AWS Bedrock by wrapping ClientError exceptions with ModelHTTPError. This is relevant for AWS Bedrock when their quota system raises ThrottlingException. Currently the ClientError exceptions are not caught, so they bypass the default fallback mechanism which expects ModelHTTPError.

from pydantic_ai import Agent
from pydantic_ai.models import FallbackModel

# Fall back to OpenAI if Bedrock throttles
fallback_model = FallbackModel(
    'bedrock:anthropic.claude-3-5-sonnet-20241022-v2:0',  # may hit rate limit
    'openai:gpt-4o',  # fallback
)

agent = Agent(fallback_model)

[DouweM]

@cjermain Thanks Colin!