fix: avoid writing None AWS region to environment

[euisuh]

Summary

Fix the manually maintained AWS client so constructing cohere.manually_maintained.cohere_aws.client.Client() without an explicit aws_region no longer writes None into AWS_DEFAULT_REGION.

When both aws_region and AWS_DEFAULT_REGION were unset, the client attempted:

os.environ["AWS_DEFAULT_REGION"] = None

which raises TypeError: str expected, not NoneType before boto3 can resolve its normal region provider chain. This change only writes the environment variable when aws_region is provided.

Test Plan

• pytest -q tests/test_aws_client_unit.py
• pytest -q tests/test_aws_client_unit.py tests/test_embed_utils.py tests/test_embed_streaming.py
• ruff check src/cohere/manually_maintained/cohere_aws/client.py tests/test_aws_client_unit.py
• git diff --check

---

Note

Low Risk
Narrow init guard around environment mutation; behavior unchanged when an explicit region is passed.

Overview
Fixes a crash when constructing cohere.manually_maintained.cohere_aws.client.Client() without aws_region and with AWS_DEFAULT_REGION unset. The client previously always assigned os.environ['AWS_DEFAULT_REGION'] = aws_region, which raised TypeError when aws_region was None. It now only sets that env var when aws_region is provided, so boto3 can use its normal region resolution chain.

Adds a unit test that clears the environment, constructs Client() with no region, and asserts AWS_DEFAULT_REGION is not set while boto3 still receives region_name=None.

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