internal/jsonrpc2: remove unused code (#910)

Remove code inherited from x/tools/internal/jsonrpc2_v2 that is not used
by MCP transports. MCP transports implement their own framing and
connection management, using only the Connection engine and message
encode/decode from this package.

Removed files:
- serve.go: Server, Listener, Dialer, idleListener, Dial, NewServer
- net.go: NetListener, NetDialer, NetPipeListener
- serve_test.go, jsonrpc2_test.go: tests for removed infrastructure

Removed from remaining files:
- frame.go: Framer interface, RawFramer, HeaderFramer implementations
- conn.go: Binder, BinderFunc, ConnectionOptions, bindConnection
- jsonrpc2.go: ErrIdleTimeout, PreempterFunc, defaultHandler, async
helper
- wire.go: ErrServerOverloaded
- conn.go: AsyncCall.IsReady (unused method)

internal/jsonrpc2/wire.go

@@ -28,9 +28,6 @@ var (
 	// The following errors are not part of the json specification, but
 	// compliant extensions specific to this implementation.
 
-	// ErrServerOverloaded is returned when a message was refused due to a
-	// server being temporarily unable to accept any new messages.
-	ErrServerOverloaded = NewError(-32000, "overloaded")
 	// ErrUnknown should be used for all non coded errors.
 	ErrUnknown = NewError(-32001, "unknown error")
 	// ErrServerClosing is returned for calls that arrive while the server is closing.