Fixes Datadog issue 5efbb1d4-eec5-11f0-8f8e-da7ad0900000
Add ExceptionGroup unwrapping in OAuth stream exception handler.
The bug was caused by ExceptionGroup not being caught by the general
`except Exception` handler, since ExceptionGroup is a subclass of
BaseException, not Exception. This caused TaskGroup errors to escape
as unhandled ExceptionGroups in Datadog.
The fix adds an explicit ExceptionGroup handler before the general
Exception handler, following the same unwrapping pattern used in
other parts of the codebase (mcp_tool_executor.py, base_client.py).
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