Add missing test coverage for asyncio.graph #148701

Code coverage analysis of Lib/asyncio/graph.py reveals several uncovered branches and error-handling blocks. We need to add tests for the following scenarios in test_asyncio/test_graph.py:

1. Invalid Types:
   • Triggering TypeError by passing a non-Future object to capture_call_graph() and _build_graph_for_future().
2. Context Errors & Fallbacks:
   • Triggering RuntimeError by calling capture_call_graph() outside of a running event loop without providing a future.
   • Returning None from capture_call_graph() (and subsequently "" from format_call_graph()) when called inside a running event loop, but not within an active task (when no future is provided).
3. The limit Parameter:
   • Testing the truncation logic using both positive and negative limit values in capture_call_graph() and _build_graph_for_future().
4. Generator Introspection:
   • Hitting the elif hasattr(coro, 'ag_await') branch in _build_graph_for_future() to test ag_await traversal.
   • Formatting standard (synchronous) generators in format_call_graph() to test the except AttributeError fallback that accesses gi_frame and gi_code.

Linked PRs

• gh-148701: Add missing test coverage for asyncio.graph #148702