Move markInitialized/reconnect after status code check in Streamable HTTP transport

[rameshreddy-adutla]

Summary

Move markInitialized() and reconnect() inside the 2xx success branch in HttpClientStreamableHttpTransport#sendMessage so they are only called when the server actually accepted the connection.

Problem

When connecting to an MCP server that doesn't support Streamable HTTP transport (returns 405 Method Not Allowed), the transport still calls markInitialized() and reconnect() before checking the status code. This triggers an unnecessary GET request, causing duplicate SSE sessions when using transport fallback (Streamable HTTP → SSE).

The root cause is in the flatMap chain at line ~480: markInitialized() and reconnect() execute unconditionally before the status code is evaluated on the next line.

Fix

Move the markInitialized()/reconnect() block inside the if (statusCode >= 200 && statusCode < 300) branch. This ensures:

• Error responses (4xx, 5xx) no longer trigger session initialization or reconnection
• Transport fallback works cleanly without duplicate sessions on the upstream server
• Success path behavior is unchanged — session is initialized and async stream opened only on 2xx

Testing

All 98 Streamable HTTP transport tests pass. The 1 pre-existing failure (startContainer) is a Docker/Testcontainers infrastructure issue unrelated to this change.

Fixes #773