Validate FastGelu fusion scale node (#32016)
This pull request improves the robustness of the FastGelu fusion
optimization by ensuring malformed nodes are properly skipped and adds a
test to verify this behavior. The main changes include stricter input
validation in the fusion logic and a new unit test.
**Fusion logic improvements:**
* Added explicit checks for the number of inputs (`InputDefs().size()`)
in `Mul` and `Pow` nodes within the `FastGeluFusion` optimizer to ensure
only well-formed nodes are considered for fusion.
[[1]](diffhunk://#diff-8f18e5c2ad33a6cc11a340f2c0ff3ce5ad63beed1dcc31ea49f1ff409ef030c9R39)
[[2]](diffhunk://#diff-8f18e5c2ad33a6cc11a340f2c0ff3ce5ad63beed1dcc31ea49f1ff409ef030c9L89-R91)
[[3]](diffhunk://#diff-8f18e5c2ad33a6cc11a340f2c0ff3ce5ad63beed1dcc31ea49f1ff409ef030c9R119)
**Testing enhancements:**
* Introduced a new test, `FastGeluFusionSkipsMalformedScaleMul`, that
modifies a model to create a malformed `Mul` node and verifies that the
fusion optimizer correctly skips it (i.e., does not produce a `FastGelu`
node).
