test: Resolve L0_backend_python test flakiness issues

[pskiran1]

What does the PR do?

File	Changes
qa/python_models/dlpack_square/model.py	Added is_cpu() check to use DLPack for GPU. Fixed output tensor name from OUTPUT0 to OUT to match the model config.
qa/L0_backend_python/async_execute/concurrency_test.py	Added tearDown() to always close the gRPC stream and client, preventing indefinite hangs when a test assertion fails before stop_stream().
qa/L0_backend_python/decoupled/decoupled_test.py	Increased execute_delay from 4s to 10s in test_decoupled_execute_cancel. The prior 1s margin between the model's first loop iteration and client cancellation was insufficient on loaded CI machines.
qa/L0_backend_python/test.sh	(1) Clean up orphaned /dev/shm/triton_python_backend_shm_region_* files at before test starts. (2) Filter stale regions in the 4MB size-validation loop so only regions from the current server are checked.
qa/L0_backend_python/examples/test.sh	Added retry logic (3 attempts) for git clone python_backend and the instance_kind example, which downloads from GitHub at runtime and is prone to transient 504/timeout errors.
qa/L0_backend_python/model_control/model_control_test.py	Timeout fix. Increased curl subprocess timeout from 10s to 30s. The server serializes model load requests, and later models in the invalid-name validation sequence were timing out on loaded CI.
qa/python_models/bls_model_loading/model.py	Modified test_load_with_file_override to accept UnicodeDecodeError alongside TritonModelException for the expected-failure path, since binary ONNX data occasionally triggers a pybind11 decode error at the bytes→string boundary. Additionally updated is model ready validation.

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• build
• ci
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• fix
• perf
• refactor
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Related PRs: triton-inference-server/python_backend#439

Where should the reviewer start?

Test plan:

• CI Pipeline ID: 54736980

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Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: #xxx