Convert billingOperationStore.ts to a discriminated union + owned transition function

[christian-byrne]

Context

From Slack thread on #14396 (xstate/statechart question for billing ops). Same pattern as #14300 (workspace-switch state machine consolidation): imperative flags/status field plus separate per-transition side-effect functions, instead of a discriminated union with one owned transition/reducer function.

Problem

src/platform/workspace/stores/billingOperationStore.ts manages 3 operation types x 4 statuses (pending/succeeded/failed/timeout), 3 different timeout thresholds, recursive polling with backoff, and toast/telemetry/dialog side effects per terminal transition.

BillingOperation has a status: OperationStatus field but is not a true discriminated union — fields like actionUrl and authenticationRequiredSeen are typed as always-present but are only meaningful while status === 'pending'. Nothing in the type system prevents a 'succeeded' or 'timeout' operation from carrying a stale actionUrl.

handleSuccess(), handleFailure(), handleTimeout() are three separate functions, each independently doing: status update, cleanup (timers/toasts), telemetry, toast, and terminal-promise resolution. No single function owns "how does this operation reach a terminal state" — the shape is repeated 3 times with small variations, easy to drift.

Proposed fix

Convert BillingOperation into a real discriminated union on status, each variant carrying only its relevant fields (e.g. actionUrl/authenticationRequiredSeen only exist on the pending variant). Replace handleSuccess/handleFailure/handleTimeout with one transition function that takes the current operation and an outcome, returns the next operation state, and the side effects (cleanup/telemetry/toast/resolve) run once, driven by the transition result rather than duplicated per outcome.

No new dependency needed (skip xstate, per Slack discussion — team consensus was bundle size isn't the blocker, it's the added concept/onboarding cost, and a plain TS union + reducer gets most of the safety).

Reference

• Same pattern/precedent: Consolidate workspace-transition sequence into one owned state machine (follow-up to #14290) #14300 (workspace-switch transition consolidation)
• Worked example of the target shape: fix: isolate drafts during workspace auth recovery #14306 (merged) — endWorkspaceSession() in workspaceAuthStore.ts is the equivalent "one owned transition" for the auth-recovery case
• Origin: Slack thread on fix: restart timed-out billing operations #14396, discussion with ShihChi/Nathaniel/DrJKL

[coderabbitai]

🔗 Related PRs

#14410 - refactor: model billing operations as an xstate machine [open]

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[christian-byrne]

Implemented and self-reviewed: #14421. All existing tests (billingOperationStore.test.ts + consumers, 989 total across src/platform/workspace) pass unmodified — the refactor is purely internal representation.

[christian-byrne]

Closing as completed via absorption rather than the original PR. #14421 (the PR that implemented this) was closed Aug 9 — its discriminated-union work was absorbed into main through 6 other billing PRs (#14490, #14605, #14705, #14310, #14910, #14216) that landed in parallel and built on the same pattern. The one piece that didn't survive (the try/finally fix for resolveTerminal not being guaranteed on throw) was extracted into #14964, which now correctly wraps cleanup/telemetry/reconciliation too per CodeRabbit's follow-up finding.

main's billingOperationStore.ts has the discriminated union, one owned transition pattern, cancel op type, workspace-scoping, and 69+ unit tests — everything this issue asked for.