fix: handle cancelled SSE streams

[noamkush]

This fixes an issue I had using the plugin, have been getting this error:

1007 |             const encoder = new TextEncoder();
1008 |             let closed = false;
1009 |             const sendSSE = (data) => {
1010 |                 if (closed)
1011 |                     return;
1012 |                 controller.enqueue(encoder.encode(`data: ${JSON.stringify(data)}\n\n`));
                 ^
TypeError: Invalid state: Controller is already closed
 code: "ERR_INVALID_STATE"

      at sendSSE (/home/noam/.cache/opencode/packages/opencode-cursor-oauth@latest/node_modules/opencode-cursor-oauth/dist/proxy.js:1012:28)
      at <anonymous> (/home/noam/.cache/opencode/packages/opencode-cursor-oauth@latest/node_modules/opencode-cursor-oauth/dist/proxy.js:1150:21)
      at <anonymous> (/home/noam/.cache/opencode/packages/opencode-cursor-oauth@latest/node_modules/opencode-cursor-oauth/dist/proxy.js:173:13)

I used an LLM to fix the issue. While I am not very familiar with typescript, the fix looks reasonable to me, and the issue did not persist after making the change.