feat: serve an MCP server card at /.well-known/mcp/server-card.json - #169

The new protocol-version advertisement is sourced from the MCP types package and the updated test checks that the expected supported version is present.

Adds public MCP server-card endpoints at /.well-known/mcp/server-card.json and /server-card, using shared registry branding metadata and CORS headers. The card advertises the Streamable HTTP remote and supported protocol versions, with end-to-end coverage for both routes.