chore(run-engine): trim assertion-echo comments, tighten fallback comment

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: d-cs

internal-packages/run-engine/src/engine/index.ts

@@ -1947,10 +1947,9 @@ export class RunEngine {
       return await query(prisma);
     } catch (e) {
       if (useReplica && e instanceof ExecutionSnapshotNotFoundError) {
-        // Expected during replica lag: the runner learned the snapshot id from the writer
-        // before the replica caught up. Serve the read from the writer instead of failing
-        // the poll. Only count/warn when the writer actually has the snapshot - a permanent
-        // miss (bogus or pruned snapshot id) is a real error, not replica lag.
+        // Replica lag: the runner learned this snapshot id from the writer before the
+        // replica caught up. Serve from the writer; only count/warn if the writer has it
+        // (a permanent miss is a real error, not lag).
         try {
           const result = await query(this.prisma);
           this.snapshotsSinceReplicaMissCounter.add(1);

internal-packages/run-engine/src/engine/tests/getSnapshotsSince.test.ts

@@ -889,7 +889,6 @@ describe("RunEngine getSnapshotsSince", () => {
         expect(result!.length).toBe(expectedSnapshots.length);
         expect(result!.map((s) => s.snapshot.id)).toEqual(expectedSnapshots.map((s) => s.id));
 
-        // The fallback fired exactly once - this counter is the prod rollout signal.
         expect(await getCounterValue("run_engine.snapshots_since.replica_miss")).toBe(1);
       } finally {
         await engine.quit();
@@ -1053,7 +1052,6 @@ describe("RunEngine getSnapshotsSince", () => {
         expect(result!.map((s) => s.snapshot.id)).not.toContain(tail.id);
         expect(result!.map((s) => s.snapshot.id)).toEqual(expectedSnapshots.map((s) => s.id));
 
-        // The since snapshot was on the replica - no fallback fired.
         expect(await getCounterValue("run_engine.snapshots_since.replica_miss")).toBe(0);
       } finally {
         await engine.quit();
@@ -1151,7 +1149,6 @@ describe("RunEngine getSnapshotsSince", () => {
         expect(expectedSnapshots.length).toBeGreaterThan(0);
         expect(result!.map((s) => s.snapshot.id)).toEqual(expectedSnapshots.map((s) => s.id));
 
-        // No replica read attempted, so no fallback could have fired.
         expect(await getCounterValue("run_engine.snapshots_since.replica_miss")).toBe(0);
       } finally {
         await engine.quit();
@@ -1251,7 +1248,6 @@ describe("RunEngine getSnapshotsSince", () => {
         expect(expectedSnapshots.length).toBeGreaterThan(0);
         expect(result!.map((s) => s.snapshot.id)).toEqual(expectedSnapshots.map((s) => s.id));
 
-        // The tx served the read directly - the fallback path never ran.
         expect(await getCounterValue("run_engine.snapshots_since.replica_miss")).toBe(0);
       } finally {
         await engine.quit();