fix(core): require explicit mock adapter construction

[besmpl]

Summary

Remove only the implicit fabricated mock fallback:

• core.NewInstance now remains empty when no registered provider yields an adapter
• RequestAdapter reports that failure instead of returning a fabricated mock adapter
• deterministic tests opt in with NewInstanceWithMock
• existing explicit software/noop registration remains available
• hal/allbackends keeps its current desktop software-backend registration behavior

The public constructors and explicit fallback providers remain available.

Why

A configured provider set that yields no adapter could previously look like successful GPU initialization because core fabricated a test adapter. That masks loader, registration, and platform-support failures.

Failing closed makes provider availability observable while preserving explicit software and mock use.

Validation

Go 1.25.12:

• go test ./core ./hal/allbackends .

Go 1.26.5:

• go test -race ./core .
• go vet ./core ./hal/allbackends .

Focused tests cover empty-provider failure and explicit mock construction.

This correction is independent of the Android backend preview.

Android preview relationship

This Android-independent prerequisite is stacked into draft #268 at exact head e97e4901ee76d9e5f587569c073b38114221c4e6. The draft will rebase it out after merge; Android runtime claims remain gated in that PR.

[besmpl]

Stacked consumer preview: #268 at exact head b11d2671e80081a191705c41c4e77e8b61472017 incorporates this PR's head e97e4901ee76d9e5f587569c073b38114221c4e6 (matching stable patch IDs).

The preview remains explicitly draft/non-mergeable and will be rebased to Android-only commits after this prerequisite lands.