fix: handle prompt abort to prevent JSON parse crash

[ivancernja]

Fixes #7946

When cancel() rejects the prompt callback, the stream closes without writing anything. SDK then crashes parsing empty body.

This catches the rejection and writes null instead.

Couldn't reproduce locally but the fix seems correct based on the code flow.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found two potentially related PRs:

1. PR fix(tui): handle JSON parse error on double-escape abort #7959: fix(tui): handle JSON parse error on double-escape abort

   • fix(tui): handle JSON parse error on double-escape abort #7959
   • Related because it also addresses JSON parse errors triggered by abort/cancel operations

2. PR fix: add error handling to RPC system to prevent crashes on session abort #7939: fix: add error handling to RPC system to prevent crashes on session abort

   • fix: add error handling to RPC system to prevent crashes on session abort #7939
   • Related because it handles crashes caused by abort scenarios in a similar system context

These PRs appear to be addressing the same or very similar issue—handling crashes that occur when prompt/session cancellation happens and results in empty or unparseable data. PR #8025 should be reviewed against these to ensure consistency or to determine if they're duplicate efforts.