.NET: Update Microsoft.Extensions.AI.* packages to 10.2.0

[Copilot]

Motivation and Context

Updating to the latest Microsoft.Extensions.AI.* packages (10.2.0) on NuGet as requested.

Description

Package updates in dotnet/Directory.Packages.props:

Package	Old Version	New Version
Microsoft.Extensions.AI	10.1.1	10.2.0
Microsoft.Extensions.AI.Abstractions	10.1.1	10.2.0
Microsoft.Extensions.AI.OpenAI	10.1.1-preview.1.25612.2	10.2.0-preview.1.26063.2

Transitive dependencies bumped to satisfy new requirements:

• Microsoft.Bcl.AsyncInterfaces 10.0.1 → 10.0.2
• Microsoft.Extensions.DependencyInjection.Abstractions 10.0.1 → 10.0.2
• Microsoft.Extensions.Logging.Abstractions 10.0.1 → 10.0.2
• System.Diagnostics.DiagnosticSource 10.0.1 → 10.0.2
• System.Text.Json 10.0.1 → 10.0.2
• System.Threading.Channels 10.0.1 → 10.0.2

Test updates:

The 10.2.0 release changed timestamp coalescing behavior in ToChatResponse() from "latest timestamp wins" to "first non-null valid timestamp wins". Updated AgentResponseUpdateExtensionsTests.cs to reflect this upstream behavioral change:

• Updated test timestamps from years 1-2 to realistic values (2024-2025)
• Changed expected assertions to match first-timestamp-wins semantics

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

Original prompt

Update to the latest versions of Microsoft.Extensions.AI.* on nuget (10.2.*)

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[Copilot]

Pull request overview

This PR updates the Microsoft.Extensions.AI.* packages from version 10.1.1 to 10.2.0 and adjusts tests to accommodate an upstream behavioral change in timestamp coalescing from "latest timestamp wins" to "first non-null valid timestamp wins".

Changes:

• Updated Microsoft.Extensions.AI packages to 10.2.0
• Bumped transitive dependencies (System.Text.Json, System.Threading.Channels, etc.) from 10.0.1 to 10.0.2
• Modified test assertions and timestamps to reflect the new "first timestamp wins" behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
dotnet/Directory.Packages.props	Updated Microsoft.Extensions.AI.* packages to 10.2.0 and transitive dependencies to 10.0.2
dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentRunResponseUpdateExtensionsTests.cs	Updated test timestamps to realistic values and changed assertions to match new "first timestamp wins" behavior

[stephentoub]

@copilot, please merge in main and address the merge conflict

[Copilot]

@copilot, please merge in main and address the merge conflict

Merged main and resolved the conflict in e476cdeb. The conflict was auto-resolved by git, and I verified the build and all 220 unit tests pass.