feat: Separate well-known agent card endpoint from MapA2A method

[Copilot]

This PR makes the .well-known/agent-card.json URL optional when hosting an A2A agent by separating it from the core JSON-RPC functionality.

Problem

Previously, the MapA2A method automatically mapped both the JSON-RPC endpoint and the well-known agent discovery endpoint:

app.MapA2A(taskManager, "/agent");
// This mapped BOTH:
// - POST /agent (JSON-RPC endpoint) 
// - GET /.well-known/agent.json (agent discovery)

This forced all A2A agents to expose discovery information, even when it wasn't desired for security or architectural reasons.

Solution

Split the functionality into two separate extension methods:

// Option 1: JSON-RPC only (no agent discovery)
app.MapA2A(taskManager, "/agent");

// Option 2: Agent discovery only
app.MapWellKnownAgentCard(taskManager, "/agent");

// Option 3: Both (maintains previous behavior)
app.MapA2A(taskManager, "/agent");
app.MapWellKnownAgentCard(taskManager, "/agent");

Changes Made

• Modified MapA2A: Removed the well-known endpoint mapping, now only handles JSON-RPC
• Added MapWellKnownAgentCard: New extension method that maps only the /.well-known/agent.json endpoint
• Updated samples: All sample applications now explicitly call both methods to preserve existing functionality
• Added comprehensive tests: New test suite validates independent operation of both methods

Benefits

• Flexibility: Developers can choose whether to expose agent discovery
• Security: Agents can be hosted without exposing discovery information
• Backward compatibility: Existing behavior is preserved by calling both methods
• Separation of concerns: JSON-RPC handling and agent discovery are now logically separate

This approach follows the pattern used by other A2A implementations and provides the granular control requested in the issue.

Fixes #105.

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[brandonh-msft]

@copilot don't use aspnet testhost; it's not net8 compatible

[Copilot]

@copilot don't use aspnet testhost; it's not net8 compatible

Fixed in commit 19c718b. Removed all usage of Microsoft.AspNetCore.TestHost and UseTestServer(). The tests now use unit testing approach that verifies endpoint registration without requiring a test server, following the same pattern as other tests in the project.

[brandonh-msft]

solved roslyn analyzer issues & tested locally w/ the Echo sample. the /echo/v1/card still gives back the card, but /.well-known/agent.json does not.

Do we want the /v1/card to be dead unless we map well-known as well, @darrelmiller ?

[darrelmiller]

Love it! Thanks.