fix: remove dead stub, add Python coverage, update Codecov docs

- Remove ShardRouter.ResolveShardKeys() placeholder (fan-out not implemented;
  single-shard path via ResolveShardKey is sufficient and already used)
- Add pytest-cov to CI python-test job; upload coverage.xml to Codecov
  with flags: python (alongside existing C# dotnet coverage upload)
- Add Codecov activation instructions to README badge section

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ritunjaym

.github/workflows/ci.yml

@@ -95,10 +95,20 @@ jobs:
         run: |
           python -m grpc_tools.protoc -I./protos --python_out=. --grpc_python_out=. protos/vector_service.proto
 
-      - name: Run pytest
+      - name: Run pytest with coverage
         working-directory: ./sidecar
         run: |
-          python -m pytest tests/ -v --tb=short --color=yes
+          pip install pytest-cov
+          python -m pytest tests/ -v --tb=short --color=yes \
+            --cov=. --cov-report=xml:coverage.xml --cov-report=term-missing \
+            --ignore=tests
+
+      - name: Upload Python coverage to Codecov
+        uses: codecov/codecov-action@v4
+        with:
+          files: ./sidecar/coverage.xml
+          flags: python
+          fail_ci_if_error: false
 
   # ══════════════════════════════════════════════════════════════════════════
   # Job 3: Docker Build (API)

README.md

@@ -4,6 +4,8 @@
 [![codecov](https://codecov.io/gh/ritunjaym/vector-catalog-service/branch/main/graph/badge.svg)](https://codecov.io/gh/ritunjaym/vector-catalog-service)
 [![License: MIT](https://img.shields.io/badge/License-MIT-yellow.svg)](https://opensource.org/licenses/MIT)
 
+> **Coverage:** C# and Python tracked via Codecov. To activate the badge: visit [codecov.io](https://codecov.io), log in with GitHub, enable this repository, then re-run CI — the badge updates automatically after the first successful upload.
+
 > **Production-ready microservice for vector search over 100M+ NYC Taxi records**
 > Demonstrates C#/.NET 8, Python gRPC, FAISS, Delta Lake, OpenTelemetry, and cloud-native architecture
 

src/VectorCatalog.Api/Services/ShardRouter.cs

@@ -37,13 +37,4 @@ public string ResolveShardKey(string? requestedShardKey, string defaultShardKey)
         return defaultShardKey;
     }
 
-    /// <summary>
-    /// For multi-shard fan-out: returns all shard keys to query in parallel.
-    /// Currently returns a single shard. Fan-out merge is not yet implemented.
-    /// </summary>
-    public IReadOnlyList<string> ResolveShardKeys(string? requestedShardKey, string defaultShardKey)
-    {
-        var key = ResolveShardKey(requestedShardKey, defaultShardKey);
-        return [key]; // Fan-out across multiple shards is not yet implemented
-    }
 }