Replace input-constraint asserts with TranslationError in converter

[gramalingam]

Replace 5 assert statements that check input AST constraints with TranslationError (via self._fail()) to provide meaningful error messages instead of bare AssertionErrors:

• Compare expressions: require single comparison operator
• Return statements: require a return value
• For loop bounds: require a function call, no keyword arguments

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
13074	1	13073	2257

View the top 1 failed test(s) by shortest run time

onnxscript.rewriter.ort_fusions.cos_sin_cache_test.TestCosSinCacheTransform::test_cos_sin_fusion_0_smollm_test_1

Stack Traces | 12.3s run time

.nox\test_onnx_ir_git\Lib\site-packages\parameterized\parameterized.py:620: in standalone_func
    return func(*(a + p.args), **p.kwargs, **kw)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
onnxscript\rewriter\ort_fusions\cos_sin_cache_test.py:50: in test_cos_sin_fusion
    new_outputs = ort_run("optimized", model, inputs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
onnxscript\rewriter\ort_fusions\_test_utils.py:19: in ort_run
    model_proto.SerializeToString(), options, providers=providers
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   MemoryError

View the full list of 1 ❄️ flaky test(s)

tests.function_libs.torch_lib.ops_test.TestOutputConsistencyFullGraphCPU::test_output_match_opinfo__logsumexp_cpu_float16

Flake rate in main: 20.00% (Passed 44 times, Failed 11 times)

Stack Traces | 0.173s run time

.../function_libs/torch_lib/ops_test.py:243: in run_test_output_match
    torch.testing.assert_close(
E   AssertionError: Tensor-likes are not close!
E   
E   Mismatched elements: 1 / 5 (20.0%)
E   Greatest absolute difference: 2.288818359375e-05 at index (1,) (up to 1e-05 allowed)
E   Greatest relative difference: 0.0022869110107421875 at index (1,) (up to 0.001 allowed)

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