Add signal contracts for reachability, exploitability, trust, and unknown symbols

- Introduced `ReachabilityState`, `RuntimeHit`, `ExploitabilitySignal`, `ReachabilitySignal`, `SignalEnvelope`, `SignalType`, `TrustSignal`, and `UnknownSymbolSignal` records to define various signal types and their properties.
- Implemented JSON serialization attributes for proper data interchange.
- Created project files for the new signal contracts library and corresponding test projects.
- Added deterministic test fixtures for micro-interaction testing.
- Included cryptographic keys for secure operations with cosign.

tests/StellaOps.Gateway.WebService.Tests/GatewayHealthTests.cs

@@ -0,0 +1,27 @@
+using Microsoft.AspNetCore.Mvc.Testing;
+using Xunit;
+
+namespace StellaOps.Gateway.WebService.Tests;
+
+public class GatewayHealthTests : IClassFixture<WebApplicationFactory<Program>>
+{
+    private readonly WebApplicationFactory<Program> _factory;
+
+    public GatewayHealthTests(WebApplicationFactory<Program> factory)
+    {
+        _factory = factory;
+    }
+
+    [Fact]
+    public async Task HealthEndpoint_ReturnsOk()
+    {
+        // Arrange
+        var client = _factory.CreateClient();
+
+        // Act
+        var response = await client.GetAsync("/health");
+
+        // Assert
+        response.EnsureSuccessStatusCode();
+    }
+}