test(storage): enable long paths for bare Git fixtures

[liugddx]

Summary

• run bare-repository test Git commands with core.longpaths=true
• match the fixed Git runtime used by the managed workspace implementation
• keep the conflicting managed head-ref fixture valid under deeply nested Windows CI temp roots

Closes the remaining Git workspace failure tracked by #2142.

Validation

• Core and Storage builds
• managed head-ref conflict test passes with TEMP/TMP nested to match the failing Actions path depth
• Biome format check
• git diff --check

[jackwener]

Adversarial review — approve

Test-only: gitBare now passes -c core.longpaths=true, matching production git-workspace-service fixed runtime. Deep Windows CI temp paths no longer break bare-repo head-ref fixtures. No production code change. LGTM.