ggml WebGPU: remove userdata from request adapter callback (#15527)

ggml WebGPU: remove userdata from request adapter callback (#15527)

* ggml WebGPU: remove userdata from request adapter callback

This commit removes the `userdata` parameter from the WebGPU request
adapter callback in `ggml-webgpu.cpp`. Instead, the lambda function
captures the `webgpu_context` directly.

The motivation for this change is to simplify the code and improve
readability.

* inline the callback lambda into the RequestAdapter call

This commit removes the callback lambda variable and inlines it directly
into the RequestAdapter call.