Added changeset

Author: monty-sei

.changeset/calm-houses-listen.md

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+---
+"@sei-js/mcp-server": patch
+---
+
+Fix session binding and response isolation in the HTTP SSE transport.
+
+- POST handler now validates `sessionId` on every request — rejects missing session IDs (400) and unknown session IDs (404)
+- Each POST is routed to the transport instance that owns the matching session ID, preventing cross-client request injection
+- Session IDs now use the MCP SDK's `transport.sessionId` rather than `Date.now()`