Add provider label metadata characterization tests

Tests/CodexBarTests/ProviderLabelMetadataCharacterizationTests.swift

@@ -0,0 +1,63 @@
+import CodexBarCore
+import Testing
+
+struct ProviderLabelMetadataCharacterizationTests {
+    // MARK: - Label non-empty constraints
+
+    @Test
+    func `displayName is non-empty for all providers`() {
+        for descriptor in ProviderDescriptorRegistry.all {
+            #expect(
+                !descriptor.metadata.displayName.isEmpty,
+                "Provider \(descriptor.id.rawValue) has empty displayName.")
+        }
+    }
+
+    @Test
+    func `sessionLabel is non-empty for all providers`() {
+        for descriptor in ProviderDescriptorRegistry.all {
+            #expect(
+                !descriptor.metadata.sessionLabel.isEmpty,
+                "Provider \(descriptor.id.rawValue) has empty sessionLabel.")
+        }
+    }
+
+    // MARK: - Known empty weeklyLabel exceptions
+
+    @Test
+    func `weeklyLabel empty providers are explicitly characterized`() {
+        // Allowlist of providers known to have empty weeklyLabel on current main.
+        // If a new provider is added with empty weeklyLabel, this test fails and
+        // requires a deliberate decision to add it here — preventing silent regressions.
+        let knownEmptyWeeklyLabelProviders: Set<UsageProvider> = [.mistral]
+        for descriptor in ProviderDescriptorRegistry.all where descriptor.metadata.weeklyLabel.isEmpty {
+            #expect(
+                knownEmptyWeeklyLabelProviders.contains(descriptor.id),
+                "Provider \(descriptor.id.rawValue) has empty weeklyLabel and is not in the known exception list.")
+        }
+    }
+
+    // MARK: - Invariant: supportsOpus implies opusLabel
+
+    @Test
+    func `supportsOpus providers declare non-empty opusLabel`() {
+        for descriptor in ProviderDescriptorRegistry.all where descriptor.metadata.supportsOpus {
+            #expect(
+                descriptor.metadata.opusLabel != nil && !descriptor.metadata.opusLabel!.isEmpty,
+                "Provider \(descriptor.id.rawValue) has supportsOpus=true but opusLabel is nil or empty.")
+        }
+    }
+
+    // MARK: - opusLabel structural constraint
+
+    @Test
+    func `opusLabel is nil or non-empty`() {
+        for descriptor in ProviderDescriptorRegistry.all {
+            if let opusLabel = descriptor.metadata.opusLabel {
+                #expect(
+                    !opusLabel.isEmpty,
+                    "Provider \(descriptor.id.rawValue) has empty opusLabel string instead of nil.")
+            }
+        }
+    }
+}