mcp: avoid panicking when a resource handler returns nil (#1176)

### What

`readResource` dereferenced the handler's result before the `if res ==
nil`
check that was meant to guard it. A `ResourceHandler` returning `(nil,
nil)`
therefore panicked instead of returning an error:

- modern clients: `handleMultiRoundTripResult` calls `res.setResultType`
on the
  nil result, and
- older clients: `res.setDefaultCacheableValues()` runs on the nil
result.

Either way the `if res == nil` guard a few lines below was dead code.

### Why it matters

Request handlers run in their own goroutine and nothing recovers, so the
panic
does not just fail the one request. It terminates the whole server
process and
every concurrent session.

The sibling handlers already avoid this: `callTool` and `getPrompt` both
guard
their result with `if err == nil && res != nil` before touching it, so a
`(nil, nil)` return flows up and becomes a normal error response.

### Change

Move the nil check in `readResource` ahead of
`handleMultiRoundTripResult` and
`setDefaultCacheableValues`, so a nil result returns the existing
descriptive
error (`read handler returned nil information`) instead of panicking.
Added a
regression test that drives a resource handler returning `(nil, nil)`
through
the server and asserts a clean error.

mcp/server.go

@@ -1035,6 +1035,9 @@ func (s *Server) readResource(ctx context.Context, req *ReadResourceRequest) (*R
 	if err != nil {
 		return nil, err
 	}
+	if res == nil {
+		return nil, fmt.Errorf("reading resource %s: read handler returned nil information", uri)
+	}
 	if err := handleMultiRoundTripResult(req.Session, s.opts.Logger, res); err != nil {
 		return nil, err
 	}
@@ -1042,7 +1045,7 @@ func (s *Server) readResource(ctx context.Context, req *ReadResourceRequest) (*R
 	if res.resultType == resultTypeInputRequired {
 		return res, nil
 	}
-	if res == nil || res.Contents == nil {
+	if res.Contents == nil {
 		return nil, fmt.Errorf("reading resource %s: read handler returned nil information", uri)
 	}
 	// As a convenience, populate some fields.