fix(agno): clean up async streams on iteration errors

[KXHXK]

Description

• handle exceptions raised after an Agno async stream has started yielding
• mark the span as ERROR, record the original exception, end the span, and remove its stored context exactly once
• re-raise the original stream exception to preserve caller behavior
• add an offline regression test for the post-yield failure path

Without this cleanup, failed async Agent/Team streams leave an unended span and a stale entry in StreamingContextManager.

Closes #1430

Testing

• uv run ruff check agentops/instrumentation/agentic/agno/instrumentor.py tests/unit/instrumentation/test_agno_instrumentor.py
• uv run ruff format --check agentops/instrumentation/agentic/agno/instrumentor.py tests/unit/instrumentation/test_agno_instrumentor.py
• uvx pre-commit run --files agentops/instrumentation/agentic/agno/instrumentor.py tests/unit/instrumentation/test_agno_instrumentor.py
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 uv run --no-sync pytest -p pytest_asyncio.plugin -p pytest_mock tests/unit/instrumentation -q (139 passed)