Prepare 2.7.0 release

[joseharriaga]

No description provided.

[Copilot]

Pull Request Overview

This PR prepares the OpenAI .NET library for the 2.7.0 release. The changes include version bumping, test recording updates to reflect API changes (notably the transition from web_search_preview to web_search tool type), and test reliability improvements.

Key Changes

• Version updated from 2.6.0 to 2.7.0 across project files
• Test session recordings updated with new API responses and tool type changes
• Test reliability improvements including timeout increases, rate limiting delays, and reduced test iteration counts

Reviewed Changes

Copilot reviewed 8 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/OpenAI.csproj	Updated VersionPrefix from 2.6.0 to 2.7.0
CHANGELOG.md	Updated release date for version 2.7.0
scripts/Test-ApiCompatibility.ps1	Updated baseline version from 2.5.0 to 2.6.0
api/OpenAI.netstandard2.0.cs	Added JsonIgnore attributes to Patch properties across multiple classes
tests/SessionRecords/*	Updated session recordings with new API responses (web_search tool type, updated timestamps, additional response fields)
tests/Responses/ResponsesToolTests.cs	Added LiveOnly attribute, increased timeouts, updated test image assets, and improved test parameters
tests/Assistants/AssistantsTests.cs	Added rate limiting delays and reduced test iteration counts from 10 to 5
tests/Assistants/Assistants.VectorStoresTests.cs	Added rate limiting delays and reduced test iteration counts for better reliability

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