.NET: Properly point agentCard to agent endpoint

[DeagleGross]

In the previous fix (#2047) I've incorrectly pointed agentCard's URL to agent card instead of the agent endpoint.
This PR fixes the AgentCard URL to correct value.

Fixes #2386

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.

[Copilot]

Pull request overview

This PR corrects a bug in the AgentCard URL assignment. The previous implementation incorrectly pointed the agentCard's URL to the card endpoint (/v1/card), when it should point to the agent endpoint itself.

Changes:

• Simplified URL assignment logic in AIAgentExtensions.cs to set the agentCard URL to the agent endpoint
• Updated test assertion to verify the URL points to the agent endpoint instead of the card endpoint

Reviewed changes

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

File	Description
dotnet/src/Microsoft.Agents.AI.Hosting.A2A/AIAgentExtensions.cs	Removed conditional logic that was incorrectly appending "/v1/card" to the agentCard URL, now correctly assigns the agent endpoint URL
dotnet/tests/Microsoft.Agents.AI.Hosting.A2A.UnitTests/A2AIntegrationTests.cs	Updated test assertion to verify agentCard URL points to agent endpoint ("/a2a/test-agent") instead of card endpoint ("/a2a/test-agent/v1/card")