feat: Implement MongoDB orchestrator storage with registry, commands, and heartbeats

- Added NullAdvisoryObservationEventTransport for handling advisory observation events.
- Created IOrchestratorRegistryStore interface for orchestrator registry operations.
- Implemented MongoOrchestratorRegistryStore for MongoDB interactions with orchestrator data.
- Defined OrchestratorCommandDocument and OrchestratorCommandRecord for command handling.
- Added OrchestratorHeartbeatDocument and OrchestratorHeartbeatRecord for heartbeat tracking.
- Created OrchestratorRegistryDocument and OrchestratorRegistryRecord for registry management.
- Developed tests for orchestrator collections migration and MongoOrchestratorRegistryStore functionality.
- Introduced AirgapImportRequest and AirgapImportValidator for air-gapped VEX bundle imports.
- Added incident mode rules sample JSON for notifier configuration.

src/Excititor/__Tests/StellaOps.Excititor.WebService.Tests/AirgapImportEndpointTests.cs

@@ -0,0 +1,54 @@
+using System.Net;
+using System.Net.Http.Json;
+using System.Text.Json;
+using StellaOps.Excititor.WebService.Contracts;
+using Xunit;
+
+namespace StellaOps.Excititor.WebService.Tests;
+
+public class AirgapImportEndpointTests : IClassFixture<TestWebApplicationFactory>
+{
+    private readonly HttpClient _client;
+
+    public AirgapImportEndpointTests(TestWebApplicationFactory factory)
+    {
+        _client = factory.CreateClient();
+    }
+
+    [Fact]
+    public async Task Import_returns_bad_request_when_signature_missing()
+    {
+        var request = new AirgapImportRequest
+        {
+            BundleId = "bundle-123",
+            MirrorGeneration = "gen-1",
+            SignedAt = DateTimeOffset.UtcNow,
+            Publisher = "mirror-test",
+            PayloadHash = "sha256:abc"
+        };
+
+        var response = await _client.PostAsJsonAsync("/airgap/v1/vex/import", request);
+
+        Assert.Equal(HttpStatusCode.BadRequest, response.StatusCode);
+        var json = await response.Content.ReadFromJsonAsync<JsonElement>();
+        Assert.Equal("AIRGAP_SIGNATURE_MISSING", json.GetProperty("error").GetProperty("code").GetString());
+    }
+
+    [Fact]
+    public async Task Import_accepts_valid_payload()
+    {
+        var request = new AirgapImportRequest
+        {
+            BundleId = "bundle-123",
+            MirrorGeneration = "gen-1",
+            SignedAt = DateTimeOffset.UtcNow,
+            Publisher = "mirror-test",
+            PayloadHash = "sha256:abc",
+            Signature = "sig"
+        };
+
+        using var response = await _client.PostAsJsonAsync("/airgap/v1/vex/import", request);
+
+        Assert.Equal(HttpStatusCode.Accepted, response.StatusCode);
+    }
+}