refactor: extract shared request-termination helper

[ubettigole]

Summary

Extract the duplicated "transition a request to a terminal state and publish its terminal log" logic into a single TerminateRequest helper in submitqueue/core/request, and route conclude, dlq, cancel, and validate through it.

Changes:

• Add corerequest.TerminateRequest(ctx, store, registry, requestID, targetState, lastError, metadata) that performs the idempotent 3-way CAS (reconcile / already-terminal / diverged / not-found), publishes the terminal RequestLog, and returns a TerminationResult{Outcome, BeforeState, AfterState} plus a separate error. Version arithmetic stays caller-side and storage.ErrVersionMismatch is passed through as retryable.
• Replace terminalStateToStatus's switch with a terminalStatusByState map lookup.
• conclude: reconcile each batch member through the helper, keeping per-outcome logs and metrics at the call site. A request referenced by the batch but missing from the store stays a hard error (retry, eventually DLQ).
• dlq: failRequest now delegates to the helper, keeping its NotFound tolerance and its reconcile/divergence log messages.
• cancel: cancelRequest delegates to the helper (Cancelling to Cancelled); the helper's re-read reports divergence cleanly instead of a spurious version-mismatch retry.
• validate: on an expected rejection (duplicate detected or custom validator failure), terminate the request to Error and ack instead of dead-lettering it; genuine infra failures still return errors.
• Add table-driven tests for TerminateRequest and update the conclude, cancel, and validate tests.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Test Plan

Issues