[WebGPU] Fix potential dangling reference in MatMul (#26126)
## Description
`MatMul` in the WebGPU EP contains a const& member initialized with a
temporary object (`Activation()`). This introduces a dangling reference
issue.
## Motivation and Context
While working on another PR, I accidentally discovered this potential
problem. Although no errors have been observed in the current code, a
dangling reference leads to undefined behavior and could cause potential
issues in the future.