fix(run-engine): don't mislabel DB errors as UnclassifiableWaitpointId in completeWaitpoint

[claude]

Changelog

WaitpointSystem.completeWaitpoint wrapped the runStore.forWaitpointCompletion(id) call in a broad try/catch that rethrew any error as UnclassifiableWaitpointId. But forWaitpointCompletion resolves the owning store by probing the database (not by classifying the id string), so a transient database/infra error surfaced from that call was being caught and mislabelled.

Before: a transient database problem (e.g. can't reach the database server under load) surfaced as a misleading UnclassifiableWaitpointId — length N matches neither cuid nor run-ops id error. This hid the real cause and lost the underlying error's type, retryability, and error-grouping.

After: the underlying database error surfaces unchanged (keeping its type, retryability, and grouping). Only a genuine id-classification failure produces UnclassifiableWaitpointId.

How: In internal-packages/run-engine/src/engine/systems/waitpointSystem.ts, the catch around forWaitpointCompletion is narrowed to convert only UnclassifiableRunId (the documented classification-failure signal — see RunStore.forWaitpointCompletion in internal-packages/run-store/src/types.ts) into UnclassifiableWaitpointId. Every other error, including DB connectivity failures, is rethrown as-is. A comment explains why DB/infra errors must not be wrapped.

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Testing

No behavior change for the existing test paths:

• The single-DB completion path returns the one store without throwing, so it never enters the narrowed catch.
• The cross-seam-guard throw tests short-circuit in the engine layer before reaching this catch and assert only that the call rejects; they remain green.

The change is confined to error classification in the store-resolution catch; the id-classification path that produces UnclassifiableWaitpointId is unchanged.

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Checklist

• The PR title follows the convention.
• I ran and tested the code works

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