[QNN EP] Support HardSigmoid (#20508)
### Description
- Adds support for float32/float16 HardSigmoid on HTP backend.
Decomposes `HardSigmoid(X)` into `max(0, min(1, alpha * X + beta))`.
- Fuses the sequence `X * HardSigmoid<alpha=1/6, beta=0.5>(X)` into a
single `HardSwish(x)`. Only applies to non-quantized HardSigmoid/Mul.
### Motivation and Context
QNN does not natively support HardSigmoid. These changes expand model
support on QNN EP.
