test(ml): improve LinearRegressor test coverage (#29083)
This pull request adds several comprehensive tests to improve the
coverage and robustness of the `LinearRegressor` operator in ONNX
Runtime. The new tests cover various input shapes, validation of
attributes, and edge cases, ensuring that the operator behaves correctly
and fails gracefully when given invalid inputs.
**Expanded test coverage for `LinearRegressor`:**
* Added tests for 1D input handling, including both valid and undersized
coefficients scenarios to verify correct computation and validation
failures.
* Added a test for zero-batch input to ensure that the operator produces
the correct output shape when there are no input samples.
* Added a test for missing intercepts to verify that the operator
correctly handles the GEMM path without bias.
* Added a test for mismatched intercepts size to confirm that intercepts
are ignored when their count does not match the number of targets.
**Edge case and error handling:**
* Added tests for input validation, including detection of coefficients
overflow and rejection of unsupported 3D input tensors.
**Test infrastructure:**
* Included necessary headers `<cstdint>` and `<limits>` to support new
test cases involving large integer values and overflow checks.
