MSBuild Server correctness validation when building VMR

## Context
When MSBuild Server is enabled by default we need confidence that VMR builds — which are the primary CI consumer — produce identical results to the non-server build.

Investigation via an agent-driven exploration surfaced a small set of fixable bugs in server behavior; nothing fundamental was identified. The primary mitigation needed for general server enablement is the restore-task workaround tracked in #13315. Other static-state concerns in SDK tasks were analyzed and do not block enablement (and remain workaround-able via `nodeReuse=false` if needed).

## Exit criteria
- VMR builds with MSBuild Server enabled produce byte-identical / functionally equivalent outputs to the non-server build.
- The restore-task workaround (#13315) is in place.
- Any small server-side bugs surfaced during validation are filed and either fixed or knowingly accepted.

## Out of scope
- Architectural questions about per-workspace server lifetime, cross-workspace caches, and the relationship between server and a future build orchestrator. Those are tracked separately.