Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

src/StellaOps.Excititor.Core/VexScoreEnvelope.cs

@@ -1,37 +1,12 @@
+using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 
 namespace StellaOps.Excititor.Core;
 
-public sealed record VexScoreEnvelope(
-    DateTimeOffset GeneratedAt,
-    string PolicyRevisionId,
-    string? PolicyDigest,
-    double Alpha,
-    double Beta,
-    double WeightCeiling,
-    ImmutableArray<VexScoreEntry> Entries)
+public sealed record VexScoreEnvelope
 {
     public VexScoreEnvelope(
-        DateTimeOffset GeneratedAt,
-        string PolicyRevisionId,
-        string? PolicyDigest,
-        double Alpha,
-        double Beta,
-        double WeightCeiling,
-        IEnumerable<VexScoreEntry> Entries)
-        : this(
-            GeneratedAt,
-            PolicyRevisionId,
-            PolicyDigest,
-            Alpha,
-            Beta,
-            WeightCeiling,
-            NormalizeEntries(Entries))
-    {
-    }
-
-    private VexScoreEnvelope(
         DateTimeOffset generatedAt,
         string policyRevisionId,
         string? policyDigest,
@@ -60,13 +35,32 @@ public sealed record VexScoreEnvelope(
             throw new ArgumentOutOfRangeException(nameof(weightCeiling), "Weight ceiling must be a finite number greater than zero.");
         }
 
-        this.GeneratedAt = generatedAt;
-        this.PolicyRevisionId = policyRevisionId.Trim();
-        this.PolicyDigest = string.IsNullOrWhiteSpace(policyDigest) ? null : policyDigest.Trim();
-        this.Alpha = alpha;
-        this.Beta = beta;
-        this.WeightCeiling = weightCeiling;
-        this.Entries = entries;
+        GeneratedAt = generatedAt;
+        PolicyRevisionId = policyRevisionId.Trim();
+        PolicyDigest = string.IsNullOrWhiteSpace(policyDigest) ? null : policyDigest.Trim();
+        Alpha = alpha;
+        Beta = beta;
+        WeightCeiling = weightCeiling;
+        Entries = entries;
+    }
+
+    public VexScoreEnvelope(
+        DateTimeOffset generatedAt,
+        string policyRevisionId,
+        string? policyDigest,
+        double alpha,
+        double beta,
+        double weightCeiling,
+        IEnumerable<VexScoreEntry> entries)
+        : this(
+            generatedAt,
+            policyRevisionId,
+            policyDigest,
+            alpha,
+            beta,
+            weightCeiling,
+            NormalizeEntries(entries))
+    {
     }
 
     public DateTimeOffset GeneratedAt { get; }
@@ -97,32 +91,9 @@ public sealed record VexScoreEnvelope(
     }
 }
 
-public sealed record VexScoreEntry(
-    string VulnerabilityId,
-    string ProductKey,
-    VexConsensusStatus Status,
-    DateTimeOffset CalculatedAt,
-    VexSignalSnapshot? Signals,
-    double? Score)
+public sealed record VexScoreEntry
 {
     public VexScoreEntry(
-        string VulnerabilityId,
-        string ProductKey,
-        VexConsensusStatus Status,
-        DateTimeOffset CalculatedAt,
-        VexSignalSnapshot? Signals,
-        double? Score)
-        : this(
-            ValidateVulnerability(VulnerabilityId),
-            ValidateProduct(ProductKey),
-            Status,
-            CalculatedAt,
-            Signals,
-            ValidateScore(Score))
-    {
-    }
-
-    private VexScoreEntry(
         string vulnerabilityId,
         string productKey,
         VexConsensusStatus status,
@@ -130,12 +101,12 @@ public sealed record VexScoreEntry(
         VexSignalSnapshot? signals,
         double? score)
     {
-        VulnerabilityId = vulnerabilityId;
-        ProductKey = productKey;
+        VulnerabilityId = ValidateVulnerability(vulnerabilityId);
+        ProductKey = ValidateProduct(productKey);
         Status = status;
         CalculatedAt = calculatedAt;
         Signals = signals;
-        Score = score;
+        Score = ValidateScore(score);
     }
 
     public string VulnerabilityId { get; }