AbortSignal is unconditionally cleared when resumableStream is enabled, making stop/cancel non-functional

[lzj960515]

Bug Description

When resumableStream is enabled on a chat request, the server-core handler unconditionally sets options.abortSignal = undefined, making it impossible for clients to cancel an in-progress generation. Clicking "Stop" in the UI only stops the client-side stream display — the backend LLM continues running to completion.

Root Cause

In packages/server-core/src/handlers/agent.handlers.ts:

```typescript
if (resumableStreamEnabled) {
options.abortSignal = undefined; // <-- AbortSignal is discarded
}
```

The comment indicates this was intentional (to support reconnection), but it has the side effect of making cancellation completely non-functional when resumable streams are in use.

Expected Behavior

Clients should be able to cancel an in-progress LLM generation even when resumableStream: true is set. The two concerns (resumability and cancellability) should be independent.

Suggested Fix

Instead of discarding the AbortSignal, the handler could:

1. Use a composite signal — create an internal AbortController for the LLM call, and abort it when either the client's signal fires or a new cancel API endpoint is called. The resumable stream adapter can then clean up independently.

2. Expose a cancel endpoint — add DELETE /agents/:id/chat/:conversationId (similar to POST /workflows/:id/executions/:executionId/cancel for workflows) that aborts the active generation and calls clearActiveStream.

Either approach decouples cancellation from the stream transport layer.

Steps to Reproduce

1. Enable resumable streams: send a chat request with options.resumableStream: true
2. Start a long-running generation
3. Call the stop/abort action (client-side AbortController.abort() or disconnect)
4. Observe: the backend LLM generation continues until completion

Environment

• VoltAgent server-core (checked against current main)
• Reproduces with any LLM provider (OpenAI, Anthropic, etc.)

Related Code

• packages/server-core/src/handlers/agent.handlers.ts — where abortSignal is cleared
• packages/server-core/src/utils/options.ts — where the signal is initially set
• packages/server-core/src/handlers/workflow.handlers.ts — workflows already have a proper cancel endpoint as a reference implementation