Add CHANGELOG entry for version 2.7.0

[Copilot]

Adds draft CHANGELOG entry for version 2.7.0 documenting changes merged since the 2.6.0 release.

Changes Documented

Features Added:

• Stable web search support via WebSearchTool and WebSearchPreviewTool classes in OpenAI.Responses
• Filename property in FileCitationMessageAnnotation
• Six reasoning streaming event types: StreamingResponseReasoningTextDeltaUpdate, StreamingResponseReasoningTextDoneUpdate, and corresponding summary events

Bugs Fixed:

• MessageCreationAttachment custom serialization (community contribution by @BenjaminDavidPinter)
• partial_image_b64 serialization name in StreamingResponseImageGenerationCallPartialImageUpdate
• Patch properties serialization issue (issue [BUG] System.InvalidOperationException System.ClientModel.Primitives.JsonPatch& #817) by adding JsonIgnore attribute to prevent System.InvalidOperationException when using JsonSerializer without the custom converter

Other Changes:

• System.ClientModel 1.7.0 → 1.8.1

Entry follows established CHANGELOG format with proper acknowledgment of community contributions.

Original prompt

1. Add a new draft entry for version 2.7.0 to the CHANGELOG.md file. It should look like this:

## 2.7.0 (Unreleased)

### Acknowledgments

### Features Added

### Bugs Fixed

### Other Changes

2. Then, go through the commits that have been merged since the release of version 2.6.0 (tag: OpenAI_2.6.0), and fill out this new 2.7.0 entry to the best of your ability.

A few things to keep in mind:

• You don't have to fill out each section of the entry. If you can't find anything reasonable or appropriate for a given section, just leave it empty.
• Use the last four entries as reference and try to match the style of the CHANGELOG.md file.

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