tests: Make billing lifecycle retries idempotent

[nishantmonu51]

Protect subscription changes and billing lifecycle workers from duplicate or partial side effects.

• add stable provider idempotency keys for plan changes and credit rollover
• finish credit transfer before changing the subscription plan
• reject private plans before starting trial side effects
• atomically claim overdue billing issues before notifications
• make duplicate and out-of-order invoice events converge monotonically
• continue processing organizations after recoverable per-org failures
• test exact timing boundaries, partial hibernation, retry, and PostgreSQL concurrency behavior

Validation:

• go test -short -race -count=1 -timeout=20m ./admin ./admin/billing ./admin/jobs/river ./admin/server

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.