docs: Add CHANGELOG.md to document API client updates, breaking changes, and new features.

Author: mardillu

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Released]
+
+### Added
+- **Assistants API (Beta)** - Full support for OpenAI Assistants v2
+  - Create, retrieve, and list Assistants
+  - Thread management (create, retrieve, delete)
+  - Message operations within threads
+  - Run operations with streaming support
+  - Vector Store support via `tool_resources`
+  - File Search and Code Interpreter v2 capabilities
+- **Files API** - Upload, list, retrieve, and delete files
+- **Fine-tuning API** - Create and manage fine-tuning jobs
+- **Streaming for Assistants** - Server-Sent Events (SSE) support via `createRunStream`
+- **Vector Store Support** - New models for `ToolResources`, `FileSearchResources`, and `VectorStore`
+- **Coroutines Support** - All API methods now use `suspend` functions
+- **OpenAiConfig** - Configuration class for API key and client settings
+
+### Changed
+- **BREAKING**: Replaced callback-based API with Kotlin Coroutines
+  - All methods now use `suspend` functions instead of callbacks
+  - Error handling via try-catch instead of error callbacks
+- **BREAKING**: Removed `OpenAiInitializer` static initialization
+  - Use `OpenAiConfig` constructor parameter instead
+- **Chat Completions** - Updated to support GPT-4 and GPT-3.5 Turbo features
+  - Function calling / Tools support
+  - JSON mode via `response_format`
+  - Added `seed`, `system_fingerprint`, and other modern parameters
+- **Image Generation** - Updated for DALL-E 3 support
+  - Added `model`, `quality`, and `style` parameters
+- **Audio API** - Enhanced transcription and translation endpoints
+  - Added optional parameters: `prompt`, `response_format`, `temperature`, `language`
+- **Tool Definition** - Made `function` nullable to support `file_search` and `code_interpreter` types
+- **Moderation API** - Updated default model to `omni-moderation-latest`
+
+### Deprecated
+- **Completions API** - Legacy text completions (use Chat Completions instead)
+- **Edits API** - Legacy edits endpoint (use Chat Completions instead)
+- `file_ids` in `CreateAssistantRequest` - Use `tool_resources` for Assistants v2
+
+### Updated
+- Gradle Wrapper to 8.6
+- Android Gradle Plugin to 8.3.1
+- Kotlin to 1.9.23
+- AndroidX Core KTX to 1.12.0
+- Material Components to 1.11.0
+- Retrofit to 2.9.0
+- OkHttp Logging Interceptor to 4.12.0
+- Added lifecycle-runtime-ktx 2.6.2 for Coroutines support
+
+### Fixed
+- Import issues in various model files
+- Compilation errors related to deprecated APIs
+
+## Migration Guide
+
+### From Callbacks to Coroutines
+
+**Before:**
+```kotlin
+OpenAiInitializer.initialize("API_KEY")
+val client = OpenApiClient()
+
+client.getChatCompletion(messages) { result, error ->
+    if (error != null) {
+        // Handle error
+    } else if (result != null) {
+        // Use result
+    }
+}
+```
+
+**After:**
+```kotlin
+val config = OpenAiConfig(apiKey = "API_KEY")
+val client = OpenApiClient(config)
+
+lifecycleScope.launch {
+    try {
+        val result = client.getChatCompletion(messages)
+        // Use result
+    } catch (e: Exception) {
+        // Handle error
+    }
+}
+```
+
+### Using Vector Stores
+
+```kotlin
+val assistant = CreateAssistantRequest(
+    model = "gpt-4",
+    tools = listOf(Tool(type = "file_search")),
+    tool_resources = ToolResources(
+        file_search = FileSearchResources(
+            vector_store_ids = listOf("vs_123")
+        )
+    )
+)
+```