handleRequest() CancellationException catch shadows stopReason:cancelled on session/cancel

[fscarponi]

handleRequest() CE catch shadows stopReason:cancelled on session/cancel

When session/cancel is received, the SDK sends a JSON-RPC error -32800 ("StandaloneCoroutine was cancelled") instead of the expected {"stopReason":"cancelled"} response.
This causes IDE clients to remain stuck in a "thinking" state after cancellation or catchAgain this pattern Client side

Expected behavior

After session/cancel, the session/prompt response should be:

{"id":5,"result":{"stopReason":"cancelled"}}

Actual behavior

The response is:

{"id":5,"error":{"code":-32800,"message":"StandaloneCoroutine was cancelled"}}

Root cause

In Agent.kt, SessionWrapper.cancel() does two things sequentially:

suspend fun cancel() {
    agentSession.cancel()                                        // step 1
    protocol.cancelPendingIncomingRequest(activePrompt.requestId) // step 2
}

Step 2 cancels the Job that runs the prompt handler. The CancellationException is caught by two competing handlers:

1. Protocol.handleRequest() (low-level) — catches CE and sends -32800
2. SessionWrapper.prompt() (high-level) — catches CE and returns
   PromptResponse(StopReason.CANCELLED)

Handler 1 always wins because it wraps the entire handler execution.
Handler 2 is effectively dead code for the cancel path — the CE never
reaches it because handleRequest() catches it first.

Reproduction

1. Start a session/prompt that triggers a long-running tool (e.g., a build command)
2. Send session/cancel while the tool is executing
3. Observe the prompt response: it's -32800 instead of stopReason:cancelled