fix: stabilize cancellation and isolation tests

[guima-why]

Summary

• guarantee the fallback optimized diagram event is enqueued before cancellation propagates
• replace the A2A isolation scenario's fixed two-second deadline with the scenario timeout
• give the cancellation regression test enough setup time to exercise cancellation deterministically
• relax two concurrent-context progress windows from one to three seconds while keeping them below the five-second fault gate
• end A2A task subscriptions immediately after emitting a terminal status, even if the SDK stream does not close

Validation

• architecture diagram cancellation regression: 10 consecutive passes
• A2A slow rollover isolation scenario: 3 consecutive passes
• concurrent-context parameterized regressions: 5 consecutive runs, 20 tests passed
• Python 3.12 push enqueue failure and adjacent subscription regressions passed
• uv run pytest -q tests/pipeline/engine/test_show_diagram_tool.py (42 passed)
• uv run pytest -q tests/a2a/test_transport_dispatcher.py (30 passed)
• targeted ruff check
• git diff --check