P2: Keep Mongo completed-action result lookup indexed

[YueZh127]

Objective

Keep every MongoDB path used by interactive completed-action polling on an indexed identity under the embedded Task.sessionActions authority introduced by #29.

Review origin and current baseline

• Original issue came from the independent review of PR github-devloop implementation for #18 #21.
• Revalidated on 2026-09-11 against qa-tools@2dd6dd27a2928f3d70fd5aad0b862117016a0aa4 after PR github-devloop implementation for #42 #43/Roll up qa-tools-fkst-staging into qa-tools #44 delivered P0: Bind terminal action retries to immutable dispatch credentials #29.
• The original premise that new actions continue accumulating in pending_actions is stale. New action and terminal result authority now live atomically in Task.sessionActions.
• This rewrite retains the real performance and query-plan defect that remains after that model change.

Current defect

MongoRepository.getSessionActionResult() first executes:

tasks.findOne({
  sessionActions: {
    $elemMatch: { id: actionId, state: 'completed' }
  }
})

There is no Task index containing sessionActions.id, so the current authoritative lookup can scan the tasks collection. SessionService.waitForResult() repeats this lookup every 250 ms until a result or deadline is reached.

The compatibility path for already-completed legacy pending_actions also queries { id: actionId, state: 'completed' }, while the legacy documents use _id=actionId and current indexes only cover in-flight migration states. That fallback can scan the legacy collection as well.

The action_results compatibility path already queries the default indexed _id and must remain readable.

Required behavior

• Resolve current embedded Task.sessionActions results through a justified indexed identity without changing Task/action authority.
• Resolve completed legacy pending_actions by indexed _id, or add a bounded and justified equivalent index.
• Preserve existing action_results._id compatibility.
• Keep result identity, immutable terminal completion, dispatch binding, one-active-action admission, and Memory/Mongo observable parity unchanged.
• Avoid creating a second authoritative result store merely to make polling convenient.

Acceptance criteria

• Add a Mongo query-plan contract for the current embedded Task result lookup and prove the winning plan does not contain COLLSCAN.
• Add query-plan contracts for action_results and completed legacy pending_actions; prove each compatibility lookup is indexed and bounded.
• Cover present, missing, wrong-state, and legacy completed results without changing the public response.
• Verify that polling by one action ID cannot return another Task's result and that terminal result immutability remains intact.
• Justify any new multikey index with its write/storage impact and ensure it supports the exact production predicate.
• Run build, typecheck, lint, worker bundle, fixture drift, full tests, standalone Mongo contracts, and all four PR checks.
• Do not use a timeout increase, longer polling interval, cache, or process-local map as the correctness fix.

Scope and dispatch gate

• This issue is a Mongo result-lookup/indexing change only.
• Do not change Talos Testing Tool/QARun/provider/deployment contracts.
• Do not mix P1: Make non-testing Task input and handoff intents restart-convergent #38 input/handoff durability, P2: Prove concurrent closing-expiry convergence across repositories #45 closing-expiry regression coverage, or P1: Make non-testing Task terminal events restart-convergent #46 terminal-event restart convergence into this work.
• Sequence after P1: Make non-testing Task input and handoff intents restart-convergent #38. Keep this issue unlabelled and unrouted until P1: Make non-testing Task input and handoff intents restart-convergent #38 is delivered and reviewed.
• Before routing, record fresh qa-tools and qa-tools-fkst-staging SHAs plus FKST package/manifest authority.
• Deliver through qa-tools-fkst-staging, followed by a separate human-reviewed integration PR into qa-tools; no direct merge, auto-merge, squash, rebase, force-push, or branch deletion.

[YueZh127]

Dependency update after PR #25: qa-tools@77732c5a95972fe702a0c43c9e2e3917113640ff contains #22's repository terminal CAS and post-merge CI is green. The required order is now #26 (lost-ack service convergence), then #23 (projection convergence), then this index remediation. Keep this issue un-routed until both predecessors merge and record a fresh qa-tools SHA before FKST dispatch.

[YueZh127]

Sequencing refinement: required order is #26 (lost-ack service convergence) -> #27 (concurrent expiry teardown idempotency) -> #23 (Task projection convergence) -> this Mongo index remediation. Keep un-routed until predecessors merge and record the exact current qa-tools SHA before dispatch.

[YueZh127]

Dependency correction after PR #28: required order is now #29 -> #27 -> #23 -> this Mongo index remediation. Keep un-routed and perform a fresh post-merge preflight after all predecessors; do not branch from partial baseline qa-tools@748a8e7 for this task.