tests: Preserve report delivery retry invariants

[nishantmonu51]

Make report delivery state explicit across partial notification failures.

• track whether any notifier has already produced an external side effect
• retry only failures that occurred before all delivery attempts remained clean
• surface missing recipient content instead of silently succeeding
• use the newest report history entry when exposing warnings
• cover schedule boundaries, watermarks, limits, and DST behavior

Validation:

• go test -short -race -count=1 -timeout=10m ./runtime/reconcilers -run '^Test(Report|ClassifyReport|NextRefresh|CalculateReport|LatestReport)'

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. No documentation update is required.
• Intend to cherry-pick into the release branch
• I am proud of this work!

[nishantmonu51]

Temporarily closing to keep the first review wave focused. We will reopen this PR after #9730, #9731, and #9732 have merged. The branch is being retained.