[eno][part1/2] Continue synthesis with incomplete ResourceSlice history

[Ruinan Liu (ruinan-liu)]

Allow synthesis to proceed when historical ResourceSlice references are nil, empty, or point to missing objects, instead of blocking on unavailable history.

• Generate desired resources and tombstones using the historical slices that remain available.
• Set  TombstoneRecoveryRequired  when history is incomplete or no current synthesis exists, and carry the flag forward across subsequent syntheses.
• Allow reconstitution to skip unavailable previous history while preserving strict loading of current desired resources and existing handling of non-404 API errors.
• Handle malformed references safely in resynthesis requests, cleanup, and not-ready diagnostics.
• Include the recovery flag in the Composition CRD schema.

This is Part 1 only: the flag does not block synthesis or reconciliation. Recovering deletions from unavailable history and clearing the flag are deferred to a later change.

Testing

Added 36 test groups with 170 subtests covering unit, executor-functional, and envtest controller-integration scenarios. All new tests pass, including race checks.