mcp: log out-of-band errors instead of dropping them (#887)

## Summary

Two error sites in the MCP runtime were previously unobservable:

- **`mcp/shared.go` `startKeepalive`** — when a keepalive ping failed,
the session was closed silently with no indication of why.
- **`mcp/transport.go` `connect`** — `jsonrpc2.OnInternalError` printed
via the package-level `log.Printf`, bypassing the `*slog.Logger`
configured on `ServerOptions`/`ClientOptions`.

Both sites now report through the existing `*slog.Logger` that `Server`
and `Client` already guarantee non-nil via `ensureLogger`. The logger is
threaded into the unexported `startKeepalive` and `connect` helpers as a
parameter — **no new public API surface**.

## Context

Per @jba's [suggestion on
#865](https://github.com/modelcontextprotocol/go-sdk/pull/865#issuecomment-4156166474):

> Maybe we can just use the logger in all the places we currently drop
the error. Then it's still reported, but we don't have to add any API.

This PR is that smaller alternative. It addresses the concrete
observability gap in #218 without expanding `ServerOptions`. PR #865
(the `ErrorHandler` callback proposal) remains open pending community
input on whether a structured callback API is also wanted on top of
logging.

`notifySessions` already logs at `Warn` level on delivery errors and is
intentionally left untouched.

## Test plan

- [x] New regression test `TestKeepAliveFailure_Logged` asserts the
keepalive failure produces a log line on the configured logger.
- [x] Verified the test fails when the new `logger.Error(...)` call is
removed (sanity check that it pins the behavior).
- [x] `go test ./mcp/... -count=1` passes.
- [x] `go vet ./...` clean.
- [x] `go build ./...` clean.

Refs #218

---------

Co-authored-by: Maciej Kisiel <mkisiel@google.com>

mcp/server.go

@@ -1026,7 +1026,7 @@ func (s *Server) Connect(ctx context.Context, t Transport, opts *ServerSessionOp
 	}
 
 	s.opts.Logger.Info("server connecting")
-	ss, err := connect(ctx, t, s, state, onClose)
+	ss, err := connect(ctx, t, s, state, onClose, s.opts.Logger)
 	if err != nil {
 		s.opts.Logger.Error("server connect error", "error", err)
 		return nil, err
@@ -1531,7 +1531,7 @@ func (ss *ServerSession) Wait() error {
 
 // startKeepalive starts the keepalive mechanism for this server session.
 func (ss *ServerSession) startKeepalive(interval time.Duration) {
-	startKeepalive(ss, interval, &ss.keepaliveCancel)
+	startKeepalive(ss, interval, &ss.keepaliveCancel, ss.server.opts.Logger)
 }
 
 // pageToken is the internal structure for the opaque pagination cursor.