fix: prevent uncaught Claude Code bridge WebSocket errors from masking startup timeouts (#19750)

## Background

A stalled Claude Code bridge WebSocket handshake could emit an uncaught
cleanup error, replacing the expected startup timeout and bypassing task
failure handling.

## Root Cause

Startup cleanup removed the WebSocket error listener before calling
`terminate()`; the reproduction confirmed that `ws` asynchronously emits
an error when terminating a CONNECTING socket, leaving that error
unhandled.

## Summary

The pull request preserves WebSocket error and close listeners through
termination cleanup, adds asynchronous termination-error regression
coverage, and includes a patch changeset for
`@ai-sdk/harness-claude-code`.

## Testing

Regression coverage verifies that terminating a connecting socket
preserves the startup timeout without an unhandled error. All 113 Claude
Code package tests and all 75 CI unit tasks pass after the latest main
merge.

## End-to-end Validation

- `pnpm -C examples/ai-functions exec tsx
src/reproduction/claude-code-connecting-cleanup-error.ts` via immutable
replay exited 0 without the original uncaught-error signal.

## Related Issues

Fixes #19731

Closes #19735

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Co-authored-by: ai-sdk-factory <308175966+ai-sdk-factory@users.noreply.github.com>
Co-authored-by: JoanLaRosa <89815407+JoanLaRosa@users.noreply.github.com>
Co-authored-by: Felix Arntz <felix.arntz@vercel.com>