tests: Make alert delivery transitions retry-safe

[nishantmonu51]

Make alert execution and notification transitions explicit across failures and cancellation.

• persist resolver and metadata failures in execution history
• retry notifier failures only when no external delivery succeeded
• prevent successful notifier deliveries from being duplicated after partial failure
• distinguish cancellation before delivery from interruption after delivery
• cover renotify boundaries, malformed persisted state, schedule limits, and URL validation
• select the newest valid alert warning history entry

Validation:

• go test -short -race -count=1 -timeout=10m ./runtime/reconcilers -run '^Test(Alert|CalculateAlert|LatestAlert|PrepareAlert|SendAlert|AddExecutionTime)'

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.