feat(demo): add model selector with API key detection and RAG parameters

Settings Panel Enhancements:
- Add provider dropdown filtered by API key availability
- Add model dropdown populated based on selected provider
- Add chunk size slider (200-2000)
- Add chunk overlap slider (0-500)
- Add similarity threshold slider (0.0-1.0)

ModelRegistry:
- Checks environment variables, system properties, and application.properties
- Filters out placeholder keys starting with "YOUR_"
- Provides list of models per provider (OpenAI, Anthropic, Azure, Ollama)

Test Fix:
- Add @BeforeAll precondition check for LangCacheIntegrationTest
- Uses JUnit Assumptions to skip when API not configured

Author: bsbodden

demos/rag-multimodal/src/main/java/com/redis/vl/demo/rag/model/ModelRegistry.java

@@ -0,0 +1,138 @@
+package com.redis.vl.demo.rag.model;
+
+import com.redis.vl.demo.rag.config.AppConfig;
+import java.util.ArrayList;
+import java.util.EnumMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Registry of available LLM providers and models.
+ *
+ * <p>Filters providers based on API key availability in environment variables
+ * or application.properties.
+ */
+public class ModelRegistry {
+
+  private static final Map<LLMConfig.Provider, List<String>> PROVIDER_MODELS = new EnumMap<>(LLMConfig.Provider.class);
+
+  static {
+    // OpenAI models
+    PROVIDER_MODELS.put(LLMConfig.Provider.OPENAI, List.of(
+        "gpt-4o",
+        "gpt-4o-mini",
+        "gpt-4-turbo",
+        "gpt-4",
+        "gpt-3.5-turbo"
+    ));
+
+    // Anthropic models
+    PROVIDER_MODELS.put(LLMConfig.Provider.ANTHROPIC, List.of(
+        "claude-3-5-sonnet-20241022",
+        "claude-3-5-haiku-20241022",
+        "claude-3-opus-20240229",
+        "claude-3-sonnet-20240229",
+        "claude-3-haiku-20240307"
+    ));
+
+    // Azure OpenAI - uses deployment names, typically configured per environment
+    PROVIDER_MODELS.put(LLMConfig.Provider.AZURE, List.of(
+        "gpt-4o",
+        "gpt-4",
+        "gpt-35-turbo"
+    ));
+
+    // Ollama - local models
+    PROVIDER_MODELS.put(LLMConfig.Provider.OLLAMA, List.of(
+        "llama3.2-vision",
+        "llama3.2",
+        "llama3.1",
+        "mistral",
+        "mixtral"
+    ));
+  }
+
+  /**
+   * Gets providers that have valid API keys configured.
+   *
+   * @return List of available providers
+   */
+  public static List<LLMConfig.Provider> getAvailableProviders() {
+    List<LLMConfig.Provider> available = new ArrayList<>();
+
+    for (LLMConfig.Provider provider : LLMConfig.Provider.values()) {
+      if (isProviderAvailable(provider)) {
+        available.add(provider);
+      }
+    }
+
+    return available;
+  }
+
+  /**
+   * Checks if a provider is available based on API key.
+   *
+   * @param provider Provider to check
+   * @return true if available
+   */
+  public static boolean isProviderAvailable(LLMConfig.Provider provider) {
+    if (!provider.requiresApiKey()) {
+      return true; // Ollama doesn't require API key
+    }
+
+    String apiKey = getApiKeyForProvider(provider);
+    return apiKey != null && !apiKey.isEmpty() && !apiKey.startsWith("YOUR_");
+  }
+
+  /**
+   * Gets API key for a provider from environment variables or application.properties.
+   *
+   * @param provider Provider
+   * @return API key or null
+   */
+  public static String getApiKeyForProvider(LLMConfig.Provider provider) {
+    return switch (provider) {
+      case OPENAI -> getApiKey("OPENAI_API_KEY", "openai.api.key");
+      case ANTHROPIC -> getApiKey("ANTHROPIC_API_KEY", "anthropic.api.key");
+      case AZURE -> getApiKey("AZURE_OPENAI_API_KEY", "azure.api.key");
+      case OLLAMA -> null; // No API key needed
+    };
+  }
+
+  private static String getApiKey(String envVar, String property) {
+    // First check environment variable
+    String value = System.getenv(envVar);
+    if (value != null && !value.isEmpty()) {
+      return value;
+    }
+    // Then check system property
+    value = System.getProperty(property);
+    if (value != null && !value.isEmpty()) {
+      return value;
+    }
+    // Finally check application.properties via AppConfig
+    AppConfig config = AppConfig.getInstance();
+    return config.getProperty(property, null);
+  }
+
+  /**
+   * Gets models available for a provider.
+   *
+   * @param provider Provider
+   * @return List of model names
+   */
+  public static List<String> getModelsForProvider(LLMConfig.Provider provider) {
+    return PROVIDER_MODELS.getOrDefault(provider, List.of(provider.getDefaultModel()));
+  }
+
+  /**
+   * Gets the default model for a provider.
+   *
+   * @param provider Provider
+   * @return Default model name
+   */
+  public static String getDefaultModel(LLMConfig.Provider provider) {
+    List<String> models = getModelsForProvider(provider);
+    return models.isEmpty() ? provider.getDefaultModel() : models.get(0);
+  }
+}