fix(test): correct async context manager mocking for bedrock routing

[iapoorv01]

Context

This PR resolves a persistent CI failure (AttributeError: __aenter__) in the test_bedrock_dynamic_args_routing test suite.

The test previously mocked the Bedrock client session using standard AsyncMock behavior, which can be unstable when resolving async context managers across different Python/Pytest environments. Because the BedrockChat production code evaluates async with self._session.client("bedrock-runtime") as client:, the mocked client must implement strict asynchronous context manager protocols. This PR explicitly constructs and injects a deterministic async context manager (__aenter__ and __aexit__) to guarantee stable test execution.

How has this been tested?

• Unit Testing: Executed local test suites against python/pathway/xpacks/llm/tests/test_llms.py to confirm the manual MagicMock correctly satisfies the async with context evaluation without raising AttributeError exceptions.
• Linting: Verified full compliance with the repository's formatting pipelines via black and flake8.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issue(s):

1. Resolves CI instability introduced by fix(llms): dynamic routing of Bedrock model-specific inference args (e.g., top_k) #248

Checklist:

• My code follows the code style of this project,
• My change requires a change to the documentation,
• I described the modification in the CHANGELOG.md file.