[MCP Authentication] Add Sub Path Protected Resource Metadata (PRM) Support (#28)

Temporarily Patch MCP .NET SDK’s `McpAuthenticationHandler`
([source](https://github.com/modelcontextprotocol/csharp-sdk/blob/ab6d3e1ac51c129d311d19afe2925e434372bbd8/src/ModelContextProtocol.AspNetCore/Authentication/McpAuthenticationHandler.cs#L57))
where the Protected Resource Metadata (PRM) URL does not include the
requested resource’s subpath.

When constructing the PRM URL, append the original request path after
the well-known suffix.
In the PRM response, set resource to the exact original resource URL
with sub path

Expected in [RFC 9728
§3.1](https://datatracker.ietf.org/doc/html/rfc9728/#section-3.1) The
PRM URL must be built by **inserting the well-known suffix between the
host and the resource’s existing path.

For a resource hosted on a subpath:

Input (client request):

```
https://exmaple.com/test/mcp
```

Expected PRM URL:

```
https://exmaple.com/.well-known/oauth-protected-resource/test/mcp
```

And in the PRM response, the `resource` value should echo the original
resource URL:

```json
{
  "resource": "https://exmaple.com/test/mcp",
  ...
}
```