Allow retrieving all registered provider IDs from registry

docs/ARCHITECTURE.md

@@ -419,6 +419,7 @@ direction LR
     namespace AiClientNamespace.Providers {
         class ProviderRegistry {
             +registerProvider(string $className) void
+            +getRegisteredProviderIds() string[]
             +hasProvider(string $idOrClassName) bool
             +getProviderClassName(string $id) string
             +isProviderConfigured(string $idOrClassName) bool

src/Providers/ProviderRegistry.php

@@ -113,6 +113,18 @@
         $this->registeredClassNames[$className] = true;
     }
 
+    /**
+     * Gets a list of all registered provider IDs.
+     *
+     * @since 0.1.0
+     *
+     * @return list<class-string<ProviderInterface> List of registered provider IDs.
+     */
+    public function getRegisteredProviderIds(): array
+    {
+        return array_keys($this->providerClassNames);
+    }
+
     /**
      * Checks if a provider is registered.
      *

tests/unit/Providers/ProviderRegistryTest.php

@@ -66,6 +66,21 @@ public function testRegisterProviderWithNonExistentClass(): void
         $this->registry->registerProvider('NonExistentProvider');
     }
 
+    /**
+     * Tests that getRegisteredProviderIds returns the correct provider IDs.
+     *
+     * @return void
+     */
+    public function testGetRegisteredProviderIds(): void
+    {
+        $this->registry->registerProvider(MockProvider::class);
+
+        $this->assertEquals(['mock'], $this->registry->getRegisteredProviderIds());
+
+        // To test with multiple providers, we would need another mock provider class.
+        // For now, this covers the basic functionality.
+    }
+
     /**
      * Tests hasProvider with unregistered provider.
      *