BedrockClientV2 SigV4 Signing Failure (Host Mismatch) and cohere_aws Dependency Issues

[q2mark]

SDK Version (required): 5.20.2

1. Bug: BedrockClientV2 SigV4 Host Header Mismatch

The BedrockClientV2 (via AwsClientV2) is currently broken for all Bedrock calls due to an incorrect host being used during the SigV4 calculation.

• The Cause: AwsClientV2 initializes with a hardcoded base_url="https://api.cohere.com". While the map_request_to_bedrock event hook updates the request.url and request.headers["host"] to the AWS endpoint, the CanonicalRequest used for signing is generated using the original api.cohere.com host.
• The Result: AWS rejects the request with a 403 Forbidden because the signature tied to api.cohere.com does not match the actual transport host (bedrock-runtime.{region}.amazonaws.com).
• Evidence: Debug logs show the CanonicalRequest host as api.cohere.com, while the final httpx dispatch is to the AWS Bedrock URL.

2. Bug: cohere_aws.Client Forced SageMaker Dependency

The manually maintained cohere.manually_maintained.cohere_aws.Client is intended to support both SageMaker and Bedrock. However, it is currently unusable for Bedrock-only users who do not have the sagemaker library installed.

• The Cause: The __init__ method of the client forcibly calls lazy_sagemaker().Session().
• The Result: This triggers an ImportError even if the user explicitly sets co.mode = Mode.BEDROCK.
• Fix Required: The SageMaker session should be initialized lazily only when Mode.SAGEMAKER is actually used.

**3. Improvement: Support for Embed v4 in cohere_aws.Client**

While BedrockClientV2 is broken, the alternative cohere_aws.Client lacks support for the new parameters introduced in Embed v4.

• The Issue: The embed method in the manually maintained client is statically defined and does not accept or pass the output_dimension parameter.
• The Result: Users cannot utilize the selectable dimensionality (256, 512, 1024) features of Embed v4 via the SDK on Bedrock.

Steps to Reproduce (BedrockClientV2)

import cohere
# This will fail with 403 Forbidden due to Host signing mismatch
co = cohere.BedrockClientV2(aws_region='us-east-1')
co.embed(texts=['test'], model='cohere.embed-v4-0', input_type='search_document')

Suggested Fixes

1. **For AwsClientV2**: Ensure the httpx.Request host and URL are fully updated prior to the SigV4Auth signer being invoked so the CanonicalRequest uses the AWS host.
2. **For cohere_aws.Client**: Guard the sagemaker.Session creation so it doesn't block users who only have boto3 and botocore installed for Bedrock use.
3. For Embed v4: Update the embed signature in the manually maintained client to support output_dimension and embedding_types.