feat(api): add fractional GPU group type

[davidLif]

Description

Stack PR 6/9 for GPU sharing migration.

Base: codex/nvfractions-readiness-status
Head: codex/fractional-gpu-group-api

This adds the FractionalGpuGroup API shape to BindRequest and generated API code. Later scheduler and Binder PRs use this typed structure instead of passing fractional GPU placement only as string group IDs.

Related Issues

N/A

Checklist

Note: Ensure your PR title follows the Conventional Commits format (e.g., feat(scheduler): add new feature)

• Self-reviewed
• Added/updated tests (if needed)
• Updated documentation (if needed)
• Added a changelog fragment via make changelog (or applied the skip-changelog label). Do not edit CHANGELOG.md directly — pending fragments are folded into it at release time.

Breaking Changes

None.

Additional Notes

Validation run:

go test ./pkg/apis/scheduling/v1alpha2 ./pkg/apis/kai/v1/...

[github-actions]

Total coverage: 54.6% -> 54.8% (delta 0.20%)

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/kai-scheduler/KAI-scheduler/pkg/apis/scheduling/v1alpha2	0.00% (ø)
github.com/kai-scheduler/KAI-scheduler/pkg/common/constants	0.00% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/kai-scheduler/KAI-scheduler/pkg/apis/scheduling/v1alpha2/bindrequest_types.go	0.00% (ø)	27 (+26)	0	27 (+26)
github.com/kai-scheduler/KAI-scheduler/pkg/apis/scheduling/v1alpha2/zz_generated.deepcopy.go	0.00% (ø)	123 (+10)	0	123 (+10)
github.com/kai-scheduler/KAI-scheduler/pkg/common/constants/constants.go	0.00% (ø)	2	0	2

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.