.NET: Add Cosmos DB implementations for ChatMessageStore and CheckpointStore. (#1838)

* draft commit

* Added Cosmos agent thread and tests

* revert unnecessary changes and fix tests

* add multi-tenant support with hierarchical partition keys (and tests).

* enhance transactional batch

* address review comments

* Address PR review comments from @westey-m

* Merge upstream/main - resolve slnx conflicts

* use param validation helpers

* Replace useManagedIdentity boolean with TokenCredential parameter

* Remove redundant suppressions and fix tests

* Rename project from Microsoft.Agents.AI.Abstractions.CosmosNoSql to Microsoft.Agents.AI.CosmosNoSql

* Refactor constructors to use chaining pattern

* Reorder deserialization constructor parameters for consistency

* Remove database/container IDs from serialized state

* Remove auto-generation of MessageId

* Optimize AddMessagesAsync to avoid enumeration when possible

* Add MaxMessagesToRetrieve to limit context window

* Make Role nullable instead of defaulting

* Fix net472 build without rebasing 19 commits

* Add Cosmos DB emulator to CI workflow

* Fix Cosmos DB emulator tests: use Skip.If instead of Assert.Fail and start emulator before unit tests

* Replace Skip.If() with conditional return to fix compilation

* Use env var to skip Cosmos tests on non-Windows CI

* Add Xunit.SkippableFact package to properly skip Cosmos tests on Linux

* Change [Fact] to [SkippableFact] for proper test skipping behavior

* Remove stale Microsoft.Agents.AI.Abstractions.CosmosNoSql directory

* Fix code formatting: add braces, this. qualifications, and final newlines

* Fix file encoding to UTF-8 with BOM, fix import ordering, and remove unnecessary using directives

* Convert backing fields to auto-properties and remove Azure.Identity using directive

* Fix CosmosChatMessageStore.cs encoding back to UTF-8 with BOM

* Fix test file formatting: indentation, encoding, imports, this. qualifications, naming conventions, and simplify new expressions

* Fix const field naming violations: Remove s_ prefix from const fields and add this. qualification to Dispose call

* Add local .editorconfig for Cosmos DB tests to suppress IDE0005 false positives from multi-targeting

* Fix IDE1006 naming violations: Rename TestDatabaseId to s_testDatabaseId and add final newlines

* Address PR review comments

Address Wesley's review comments:

- Remove Cosmos DB package references from core projects

- Delete duplicate test files from old package structure

- Remove redundant parameter validation from extension methods

Address Kiran's review comments:

- Remove redundant 429 retry logic (SDK handles automatically)

- Add explicit RequestEntityTooLarge error handling

- Remove dead code in GetMessageCountAsync

- Add defensive partition key validation comments

* Fix IDE0001 formatting error in AgentProviderExtensions.cs. Use type alias to resolve namespace conflict between Azure.AI.Agents.Persistent.RunStatus and Microsoft.Agents.AI.Workflows.RunStatus. This eliminates the need for global:: qualifier which triggered the formatter warning.

* Update package versions for Aspire 13.0.0 compatibility

* Fix TargetFrameworks in Cosmos DB projects

- Replace  with  which is defined in Directory.Build.props
- Fix package reference from System.Linq.Async to System.Linq.AsyncEnumerable to match Directory.Packages.props

* Remove redundant counter, add partition key validation, use factory pattern for deserialization

.github/workflows/dotnet-build-and-test.yml

@@ -124,6 +124,16 @@ jobs:
           popd
           rm -rf "$TEMP_DIR"
 
+      # Start Cosmos DB Emulator for Cosmos-based unit tests (only on Windows)
+      - name: Start Azure Cosmos DB Emulator
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          Write-Host "Launching Azure Cosmos DB Emulator"
+          Import-Module "$env:ProgramFiles\Azure Cosmos DB Emulator\PSModules\Microsoft.Azure.CosmosDB.Emulator"
+          Start-CosmosDbEmulator -NoUI -Key "C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw=="
+          echo "COSMOS_EMULATOR_AVAILABLE=true" >> $env:GITHUB_ENV
+
       - name: Run Unit Tests
         shell: bash
         run: |
@@ -143,6 +153,10 @@ jobs:
               echo "Skipping $project - does not support target framework ${{ matrix.targetFramework }} (supports: $target_frameworks)"
             fi
           done
+        env:
+          # Cosmos DB Emulator connection settings
+          COSMOSDB_ENDPOINT: https://localhost:8081
+          COSMOSDB_KEY: C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw==
 
       - name: Log event name and matrix integration-tests
         shell: bash 
@@ -181,6 +195,9 @@ jobs:
             fi
           done
         env:
+          # Cosmos DB Emulator connection settings
+          COSMOSDB_ENDPOINT: https://localhost:8081
+          COSMOSDB_KEY: C2y6yDjf5/R+ob0N8A7Cgv30VRDJIWEHLM+4QDU5DE2nQ9nDuVTqobD4b8mGGyPMbIZnqyMsEcaGQy67XIw/Jw==
           # OpenAI Models
           OpenAI__ApiKey: ${{ secrets.OPENAI__APIKEY }}
           OpenAI__ChatModelId: ${{ vars.OPENAI__CHATMODELID }}