feat(langchain): support async invoke & stream in langchain integration

[shellmayr]

• Support for ainvoke and astream in langchain integration

Closes TET-1019

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
24132	2	24130	2286

View the top 2 failed test(s) by shortest run time

tests.integrations.openai_agents.test_openai_agents::test_client_span_custom_model

Stack Traces | 0.002s run time

.../integrations/openai_agents/test_openai_agents.py:85: in test_agent_custom_model
    return Agent(
<string>:19: in __init__
    ???
.tox/py3.13-openai_agents-v0.2.9/lib/python3.13.../site-packages/agents/agent.py:284: in __post_init__
    raise TypeError(
E   TypeError: Agent model must be a string, Model, or None, got MagicMock

tests.integrations.openai_agents.test_openai_agents::test_client_span_custom_model

Stack Traces | 0.003s run time

.../integrations/openai_agents/test_openai_agents.py:85: in test_agent_custom_model
    return Agent(
<string>:19: in __init__
    ???
.tox/py3.10-openai_agents-v0.2.9/lib/python3.10.../site-packages/agents/agent.py:284: in __post_init__
    raise TypeError(
E   TypeError: Agent model must be a string, Model, or None, got MagicMock

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[linear]

TET-1019 Fix astream & ainvoke in Langchain Python SDK integration