feat(vcr): add experimental VCR test system for LLM/embedding recording

Implements a VCR (Video Cassette Recorder) test system that enables recording
and replaying LLM and embedding API calls for deterministic, fast, and
cost-effective testing.

Core components:
- VCRMode: 6 modes (PLAYBACK, RECORD, RECORD_NEW, RECORD_FAILED,
  PLAYBACK_OR_RECORD, OFF) with smart mode selection
- VCRExtension: JUnit 5 extension with full lifecycle callbacks
- VCRContext: Redis container management with AOF/RDB persistence
- VCRRegistry: Test recording status tracking with smart mode logic
- @VCRTest, @VCRRecord, @VCRDisabled: Annotations for test configuration

Key features:
- Redis-based cassette storage with persistence to src/test/resources/vcr-data
- Testcontainers integration for isolated Redis instances
- Call counter management for deterministic key generation
- Statistics tracking (cache hits, misses, API calls)
- Configurable data directory and Redis image

Also includes:
- Comprehensive documentation in docs/experimental section
- README section with quick start example
- Design documents for VCR system and EmbeddingsCache enhancement

All 39 VCR tests passing. LLM/embedding interceptor implementation pending.

Author: bsbodden

README.md

@@ -376,6 +376,34 @@ System.out.println(match.getDistance()); // Output: 0.273891836405
 
 > Learn more about [semantic routing](https://redis.github.io/redis-vl-java/redisvl/current/semantic-router.html).
 
+## 🧪 Experimental: VCR Test System
+
+RedisVL includes an experimental VCR (Video Cassette Recorder) test system for recording and replaying LLM/embedding API calls. This enables:
+
+- **Deterministic tests** - Replay recorded responses for consistent results
+- **Cost reduction** - Avoid repeated API calls during test runs
+- **Speed improvement** - Local Redis playback is faster than API calls
+- **Offline testing** - Run tests without network access
+
+```java
+import com.redis.vl.test.vcr.VCRTest;
+import com.redis.vl.test.vcr.VCRMode;
+
+@VCRTest(mode = VCRMode.PLAYBACK_OR_RECORD)
+public class MyLLMTest {
+
+    @Test
+    void testLLMResponse() {
+        // First run: Records API response to Redis
+        // Subsequent runs: Replays from Redis cassette
+        String response = myLLMService.generate("What is Redis?");
+        assertNotNull(response);
+    }
+}
+```
+
+> Learn more about [VCR testing](https://redis.github.io/redis-vl-java/redisvl/current/vcr-testing.html).
+
 ## 🚀 Why RedisVL?
 
 In the age of GenAI, **vector databases** and **LLMs** are transforming information retrieval systems. With emerging and popular frameworks like [LangChain4J](https://github.com/langchain4j/langchain4j) and [Spring AI](https://spring.io/projects/spring-ai), innovation is rapid. Yet, many organizations face the challenge of delivering AI solutions **quickly** and at **scale**.

core/build.gradle.kts

@@ -65,6 +65,16 @@ dependencies {
     // Cohere Java SDK for reranking
     compileOnly("com.cohere:cohere-java:1.8.1")
 
+    // VCR Test Utilities (optional - users include what they need for testing)
+    // JUnit 5 for extension development
+    compileOnly("org.junit.jupiter:junit-jupiter-api:5.10.2")
+    // Testcontainers for Redis persistence
+    compileOnly("org.testcontainers:testcontainers:1.19.7")
+    compileOnly("org.testcontainers:junit-jupiter:1.19.7")
+    // ByteBuddy for method interception (future LLM interceptor)
+    compileOnly("net.bytebuddy:byte-buddy:1.14.12")
+    compileOnly("net.bytebuddy:byte-buddy-agent:1.14.12")
+
     // Test dependencies for LangChain4J (include in tests to verify integration)
     testImplementation("dev.langchain4j:langchain4j:0.36.2")
     testImplementation("dev.langchain4j:langchain4j-open-ai:0.36.2")
@@ -79,6 +89,12 @@ dependencies {
     // Additional test dependencies
     testImplementation("com.squareup.okhttp3:mockwebserver:4.12.0")
     testImplementation("org.mockito:mockito-core:5.11.0")
+
+    // VCR test dependencies (to test VCR functionality)
+    testImplementation("org.testcontainers:testcontainers:1.19.7")
+    testImplementation("org.testcontainers:junit-jupiter:1.19.7")
+    testImplementation("net.bytebuddy:byte-buddy:1.14.12")
+    testImplementation("net.bytebuddy:byte-buddy-agent:1.14.12")
 }
 
 // Configure test execution