Add session_id to context documentation

Fast follow to #881 to document the new session_id property.

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Co-Authored-By: Claude <noreply@anthropic.com>

docs/servers/context.mdx

@@ -293,6 +293,7 @@ async def request_info(ctx: Context) -> dict:
 
 - **`ctx.request_id -> str`**: Get the unique ID for the current MCP request
 - **`ctx.client_id -> str | None`**: Get the ID of the client making the request, if provided during initialization
+- **`ctx.session_id -> str | None`**: Get the MCP session ID for session-based data sharing (HTTP transports only)
 
 ### Advanced Access