[CPU] Fix output saturation in int8 QLinearSoftmax (#29728)
### Description
Uses the correct int8 range to clamp/saturate the output of the int8
specialization of the QLinearSoftmax kernel.
Previously there was a copy-paste error, and it was saturating to 255
like in the uint8 specialization.
https://github.com/microsoft/onnxruntime/blob/main/docs/ContribOperators.md#com.microsoft.QLinearSoftmax
This change also adds a unit test case that causes output saturation,
and reproduced the bug.
### Motivation and Context
When there are:
1. Softmax outputs very close to 1 (in the original float graph), and
2. a y_scale of 1.0 / 256.0 and y_zero_point of -128 (as set by
https://github.com/microsoft/onnxruntime/blob/main/onnxruntime/python/tools/quantization/operators/softmax.py)
then the output of int8 QLinearSoftmax will saturate. Incorrect clamping
is causing this to wrap around to -128 instead, producing incorrect
results.
https://github.com/microsoft/onnxruntime/issues/29727
