feat: allow iam mcp client to take a botocore credentials object

[wzxxing]

Summary

Changes

Please provide a summary of what's being changed

Summary

Adds an optional credentials parameter to aws_iam_streamablehttp_client() that accepts botocore Credentials objects, enabling programmatic credential management without requiring boto3 session creation.

Motivation

When integrating MCP servers into AI frameworks, users may already have AWS credentials from existing boto3 sessions or credential providers. Creating a new boto3 session is unnecessary overhead and limits flexibility in credential management scenarios such as:

• Using credentials from existing boto3 clients/sessions
• Implementing custom credential rotation logic
• Testing with mock credentials
• Cross-account access patterns with assumed roles

User experience

Please share what the user experience looks like before and after this change

API Changes

Added new optional parameter to aws_iam_streamablehttp_client():

• credentials: Optional[Credentials] - When provided, takes precedence over aws_profile

Behavior

When credentials is provided:

• Bypasses boto3.Session creation entirely
• Requires aws_region to be explicitly specified (raises ValueError if missing)
• Uses provided credentials directly for SigV4 signing

When credentials is NOT provided:

• Maintains existing behavior (creates boto3.Session with profile/region)
• Backward compatible with all existing code

Example Usage

from botocore.credentials import Credentials
from mcp_proxy_for_aws.client import aws_iam_streamablehttp_client

# Use existing credentials
creds = boto3.Session().get_credentials()

async with aws_iam_streamablehttp_client(
    endpoint="https://example.amazonaws.com/mcp",
    aws_service="bedrock-agentcore",
    aws_region="us-east-1",
    credentials=creds
) as (read, write, session_id_callback):
    # Use the client
    pass

Checklist

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• I have reviewed the contributing guidelines
• I have performed a self-review of this change
• Changes have been tested
• Changes are documented

Is this a breaking change? (Y/N)

• Yes
• No

Please add details about how this change was tested.

• Did integration tests succeed?
• If the feature is a new use case, is it necessary to add a new integration test case?

Acknowledgment

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