tests: Harden deployment reconciliation and health checks

[nishantmonu51]

Cover and harden deployment worker state transitions and failure boundaries.

• include retryable jobs in reconciliation uniqueness
• schedule one follow-up when desired state changes during reconciliation
• repeat only idempotent lifecycle operations after ambiguous failures
• paginate deployment health checks without skipping or duplicating rows
• deduplicate runtime hosts and handle deletion races
• report only confirmed missing instances
• test orphan timing boundaries, primary switches, and teardown failures

Validation:

• go test -short -race -count=1 -timeout=15m ./admin/jobs/river -run '^Test(ReconcileDeployment|DeploymentsHealthCheck|DeploymentHealth|ValidateDeployments)'

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.