fix: raise ValueError when model is None in Bedrock client

[MaxwellCalkin]

Summary

When using the Bedrock client, if model is not provided in the request body, options.json_data.pop("model", None) returns None. This None is then passed to str(), producing the literal string "None", which gets URL-encoded and used in the Bedrock API URL (e.g., /model/None/invoke). This causes a confusing error from the Bedrock API rather than a clear client-side error.

Before this fix:

# model not provided → silent conversion to string "None"
# API call to /model/None/invoke → confusing Bedrock error

After this fix:

# model not provided → immediate ValueError: "model is required for Bedrock API calls"

Changes

• Added an explicit None check after popping the model from json_data in _prepare_options() in src/anthropic/lib/bedrock/_client.py
• Raises ValueError with a clear message when model is missing

Context

The Vertex client (src/anthropic/lib/vertex/_client.py) already handles this correctly by using .pop("model") without a default value, which raises a KeyError if model is missing. This fix brings the Bedrock client in line with that behavior by providing a clear, descriptive error.

Test plan

• Verified the fix raises ValueError when model is None
• Verified normal operation is unaffected when model is provided
• Confirmed the Vertex client already handles this case (no default value on .pop())