Proposal: allow `mcp.streamableClientConn` to listen to SSE stream without using `mcp.Client`

Is your feature request related to a problem? Please describe.
We use the SDK to implement reverse proxies for MCP servers and transport conversions. For that purpose, we cannot use mcp.Client directly. We use StreamableServerTransport instead to process the messages.

However, there is no exposed function for StreamableServerTransport (or streamableClientConn) to start to listen the SSE stream (the GET call), without using mcp.Client.

Describe the solution you'd like
Have a boolean say ConnectSSEOnInitializeSuccess in StreamableServerTransport to enable the listening functionality. If flag is set, c.connectStandaloneSSE() will be called automatically after receiving successful initialize result.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Expose sessionUpdated or connectStandaloneSSE() and allow external caller to trigger it.

Additional context
Add any other context or screenshots about the feature request here.

Start by reading StreamableServerTransport and streamableClientConn, then trace the successful initialize result and the existing connectStandaloneSSE path. The change is done when a transport used without mcp.Client can begin listening to the SSE stream through the agreed option or exposed trigger, with the relevant transport behavior covered by tests.