Update STS client to use new async config and fix failing container resolver tests

[arandito]

Description

This PR makes two small runtime package updates:

aws-credentials-sts: adopts new async config API

Updates AssumeRoleCredentialsResolver to build its AsyncSTSClient from AsyncSTSConfig.resolve(...) instead of the deprecated synchronous Config. Config overrides (source credentials resolver, region, and optional transport) are now assembled into a dict and resolved asynchronously, with transport only set when an HTTP client is provided.

Warning

This depends on the parallel smithy-python change that deprecates the old Config in favor of AsyncSTSConfig (smithy-lang/smithy-python#758). It must be sim-shipped in the same release as that PR; releasing independently will break aws-credentials-sts (ImportError: cannot import name 'AsyncSTSConfig').

aws-credentials-http: normalize empty query strings

Fixes ContainerCredentialsResolver to pass query=parsed.query or None when resolving from AWS_CONTAINER_CREDENTIALS_FULL_URI. urlparse yields '' for a query-less URL, and the current URI no longer normalizes that to None, so the constructed request URI didn't match expectations.

Testing

Ran the container credentials resolver unit tests standalone:

cd packages/aws-credentials-http
uv run --no-project --python 3.12 \
  --with pytest --with pytest-asyncio \
  --with-editable . \
  python -m pytest tests/unit/test_resolvers.py -o asyncio_mode=auto

All 12 tests pass, including the four test_resolver_env_* cases that previously failed on the empty-query-string mismatch.

9 aws-credentials-sts resolver tests currently fail with ImportError: cannot import name 'AsyncSTSConfig' from 'aws_sdk_sts.config' until smithy-python#758 ships the new async config. Nothing to do here until that lands; they'll pass once the two are released together.

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