.NET: fix: align Anthropic Extensions AI version

[danyalahmed1995]

Summary

Related to #5707.

Addresses the Anthropic side of a regression where provider usage could fail with MissingMethodException around Microsoft.Extensions.AI.WebSearchToolResultContent.Results.

This PR does not change the repo-pinned Google.GenAI dependency. During investigation, the latest available Google.GenAI package I found was 1.6.1, and it still appeared to reference WebSearchToolResultContent.set_Results, so the Google.GenAI side likely needs an upstream package update or a broader dependency strategy.

Changes

• Bumped Anthropic from 12.13.0 to 12.20.0
• Re-enabled Anthropic integration coverage that was disabled for the binary incompatibility
• Confirmed Anthropic 12.20.0 no longer references the removed Results accessor

Validation

• git diff --check HEAD~1..HEAD
• Metadata scan confirmed Anthropic 12.20.0 no longer references the removed WebSearchToolResultContent.Results accessor
• Confirmed the repo-pinned Google.GenAI 1.6.0 is not changed by this PR
• Separately checked the latest available Google.GenAI package found during investigation, 1.6.1, and it still appears to reference WebSearchToolResultContent.set_Results
• dotnet restore blocked locally because repo requires .NET SDK 10.0.200; installed SDK is 9.0.313
• dotnet build blocked locally for the same SDK reason
• dotnet test blocked locally for the same SDK reason

Related to #5707

[Copilot]

Pull request overview

Fixes a .NET provider regression (see #5707) by aligning the Anthropic NuGet dependency with the current Microsoft.Extensions.AI surface area and re-enabling the Anthropic integration coverage that was previously suppressed due to a binary incompatibility.

Changes:

• Bumped central package version for Anthropic from 12.13.0 to 12.20.0.
• Re-enabled Anthropic Skills integration tests by restoring the normal integration test category trait.
• Removed the hard Assert.Skip(...) that was preventing Anthropic integration fixture initialization from running at all.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
dotnet/tests/AnthropicChatCompletion.IntegrationTests/AnthropicSkillsIntegrationTests.cs	Re-enables Anthropic Skills integration tests by switching the trait back to Integration.
dotnet/tests/AnthropicChatCompletion.IntegrationTests/AnthropicChatCompletionFixture.cs	Removes the blanket skip so Anthropic integration tests can execute again (and still skip when config is missing).
dotnet/Directory.Packages.props	Updates the centrally-managed Anthropic package version to 12.20.0.

[danyalahmed1995]

Oh sorry.The repo currently pins Google.GenAI 1.6.0; the 1.6.1 note came from checking the latest available package during investigation, not from a change in this PR.I updated the PR description to distinguish the repo pinned version from the separately checked latest available version. This PR remains Anthropic-only.cked latest available version. This PR remains Anthropic only.