Add CHANGELOG entry for version 2.7.0

Co-authored-by: joseharriaga <[email protected]>

Author: Copilot

CHANGELOG.md

@@ -1,5 +1,36 @@
 # Release History
 
+## 2.7.0 (Unreleased)
+
+### Acknowledgments
+
+Thank you to our developer community members who helped to make the OpenAI client library better with their contributions to this release:
+
+- Benjamin Pinter _([GitHub](https://github.com/BenjaminDavidPinter))_
+
+### Features Added
+
+- OpenAI.Responses:
+  - Enabled support for stable web search functionality, with a separate factory for preview web search features.
+  - Added `FileName` property to `FileCitationMessageAnnotation` to provide the name of the cited file.
+  - Exposed reasoning streaming events, including:
+    - `StreamingResponseReasoningTextDeltaUpdate` and `StreamingResponseReasoningTextDoneUpdate` for reasoning text events.
+    - `StreamingResponseReasoningSummaryPartAddedUpdate` and `StreamingResponseReasoningSummaryPartDoneUpdate` for reasoning summary part events.
+    - `StreamingResponseReasoningSummaryTextDeltaUpdate` and `StreamingResponseReasoningSummaryTextDoneUpdate` for reasoning summary text events.
+
+### Bugs Fixed
+
+- OpenAI.Assistants:
+  - Fixed an issue with custom serialization for `MessageCreationAttachment` that prevented proper handling of file attachments in message creation. _(A community contribution, courtesy of [BenjaminDavidPinter](https://github.com/BenjaminDavidPinter))_
+- OpenAI.Responses:
+  - Fixed the generated serialization name of `partial_image_b64` to ensure correct serialization of partial image data in streaming responses.
+
+### Other Changes
+
+- Updated the `System.ClientModel` dependency to version 1.8.0.
+- Updated the `@typespec/http-client-csharp` generator to version 1.0.0-alpha.20251031.1.
+- Removed the preview version badge from the README file.
+
 ## 2.6.0 (2025-10-31)
 
 ### Acknowledgments