fix(tern): cancel orphaned pending tasks so they stop blocking new applies

[aparajon]

What broke

A pending task whose apply already reached a terminal state is orphaned: terminal applies are never claimed again, so no drive will ever start the task. The conflict check that guards new dispatches ("is a schema change already in progress?") saw the non-terminal task and refused — permanently. Every dispatch to that database failed with "schema change already in progress", and nothing in the system could ever clear it.

The fix

The conflict check now recognizes this shape and self-heals. Only a pending task qualifies — pending provably means no engine work or checkpoint ever existed. Anything uncertain keeps blocking.

new dispatch ── conflict check finds a non-terminal task
        │
        ├── task is pending AND its apply is terminal ──► cancel the orphan
        │                                                 (durably, + apply-log
        │                                                 entry) and proceed
        │
        └── anything else ──────────────────────────────► keep blocking
            (apply still active, stopped/retryable         (fail closed)
            task with a checkpoint, storage error,
            missing apply row)

Stopped and failed-retryable tasks are untouched — they own engine checkpoints and an operator retry budget. Databases already blocked by past orphans unblock automatically on their next dispatch, with the cancellation recorded in the owning apply's durable logs.

Completes the set with #749 (sharded dispatches drive their DDL) and #748 (a drive that can't load its tasks refuses to complete): #749 fixes the cause, #748 makes any recurrence loud, and this PR clears the blockage left behind.

🤖 Generated with Claude Code (Claude Fable 5)

[Copilot]

Pull request overview

Fixes a dispatch deadlock in the local tern client where a pending task whose parent apply is already terminal can never be driven again, but still causes the “schema change already in progress” conflict gate to block all future applies on that database. The PR adds a self-healing path that cancels only this provably-unstarted orphan shape and proceeds, while continuing to fail-closed on any uncertainty.

Changes:

• Cancel orphaned pending tasks during conflict detection when their parent apply is already in a terminal state.
• Add unit tests covering terminal/active/uncertain apply-lookup cases for orphan resolution.
• Add an integration test asserting the orphan is durably cancelled, logged to apply_logs, and does not touch the engine.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
pkg/tern/local_apply.go	Adds orphaned-pending-task cancellation to the conflict-check path.
pkg/tern/local_apply_conflict_test.go	Adds unit tests validating orphan cancellation and fail-closed behavior on uncertainty.
pkg/tern/local_orphaned_task_integration_test.go	Adds an integration test to prove durable cancellation + apply-log recording without engine interaction.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.