MR-794 step 2: address PR #68 follow-up review (Cubic) — pending dedupe + projection guard + CI

Three new findings from Cubic on commit 3223b51:

* **Pending edge cardinality counted within-input duplicates** (P2):
  count_src_per_edge's pending walk added every row to the count,
  including duplicate rows that finalize will collapse via
  dedupe_merge_batches_by_id. A LoadMode::Merge with the same edge id
  twice would over-count → spurious @card violation. Fix: when
  dedupe_key_column is Some, walk pending in reverse, track seen keys
  via HashSet, count only the kept (last-occurrence) rows. Mirrors
  finalize-time dedupe so cardinality counts what stage_merge_insert
  actually publishes.

* **scan_with_pending silently disabled merge-shadow when projection
  omitted key_column** (P2): if a caller passed Some("id") as
  key_column but their projection didn't include "id", the
  filter_out_rows_where_string_in helper passed batches through
  unchanged — silently degrading to union semantics. Fix: validate
  up front that projection contains key_column when both are Some;
  return a typed Lance error otherwise. Tightened the helper too:
  missing column is now an internal error (was a silent passthrough).

* **Cascade-vs-explicit delete test was too weak** (P2): asserted
  only that edge count decreased after delete. The cascade alone
  could satisfy that even if the explicit second-delete silently
  no-op'd. Strengthened: assert post_knows == 0, which only holds
  when both ops landed (Bob→Diana would survive if op-2 no-op'd).

CI gap: also added test_failpoints_feature job to .github/workflows/ci.yml.
The workspace test runs without --features failpoints (the feature is
behind a Cargo flag), so the failpoints test suite was never exercised
by CI before now. The new job builds + runs
`cargo test -p omnigraph-engine --features failpoints --test failpoints`
on every full CI run, mirroring the test_aws_feature pattern.

New tests on tests/runs.rs:

* load_merge_mode_dedupes_within_pending_for_cardinality_count
  (Cubic P2 #2 — pending-vs-pending dedup, distinct from the
  load_merge_mode_dedupes_edge_for_cardinality_count test which
  covers committed-vs-pending dedup).
* scan_with_pending_rejects_key_column_missing_from_projection
  (Cubic P2 #3 — verifies the up-front validation rejects bad
  callers and that the happy path still works correctly).

Local test results:

* tests/runs.rs: 23/23 passed
* tests/failpoints.rs --features failpoints: 7/7 passed (includes the
  two new finalize→publisher residual tests landed in 3223b51).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -239,6 +239,48 @@ jobs:
         if: needs.classify_changes.outputs.run_full_ci == 'true'
         run: cargo test --locked -p omnigraph-server --features aws
 
+  test_failpoints_feature:
+    name: Test omnigraph-engine --features failpoints
+    needs: classify_changes
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    permissions:
+      contents: read
+    env:
+      CARGO_TERM_COLOR: always
+    steps:
+      - name: Skip for text-only changes
+        if: needs.classify_changes.outputs.run_full_ci != 'true'
+        run: echo "Text-only change detected; skipping failpoints feature build."
+
+      - name: Checkout source
+        if: needs.classify_changes.outputs.run_full_ci == 'true'
+        uses: actions/checkout@v5.0.1
+
+      - name: Install system dependencies
+        if: needs.classify_changes.outputs.run_full_ci == 'true'
+        run: |
+          sudo apt-get update
+          sudo apt-get install -y protobuf-compiler libprotobuf-dev
+
+      - name: Install Rust stable
+        if: needs.classify_changes.outputs.run_full_ci == 'true'
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          toolchain: stable
+
+      - name: Cache Rust build data
+        if: needs.classify_changes.outputs.run_full_ci == 'true'
+        uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: |
+            . -> target
+          key: failpoints-feature
+
+      - name: Run failpoints test suite
+        if: needs.classify_changes.outputs.run_full_ci == 'true'
+        run: cargo test --locked -p omnigraph-engine --features failpoints --test failpoints
+
   rustfs_integration:
     name: RustFS S3 Integration
     needs: