Claude models fail with missing finish_reason after custom tool call completion

## Summary

Claude models (e.g. `claude-opus-4.6`) fail with "Failed to get response from the AI model; retried 5 times" after a custom tool call completes successfully. GPT models work fine with the same code. The issue is 100% reproducible.

## Root cause from server logs

The CLI subprocess server log (`~/.copilot/logs/process-*.log`) shows:

```
StreamingErrorRetryProcessor: Non-API error detected (retry 1): missing finish_reason for choice 0
...
StreamingErrorRetryProcessor: Non-API error detected (retry 6): missing finish_reason for choice 0
Ignoring event of kind: turn_failed
```

The server log also shows:
```
"WEBSOCKET_RESPONSES": "false",
"copilot_cli_websocket_responses": "null",
```

This suggests the SDK CLI subprocess is being routed through the **Chat Completions API** path (not the Responses API). The Copilot proxy's Claude → Chat Completions format adapter appears to not set `finish_reason` in the streaming response after tool results are submitted, causing `StreamingErrorRetryProcessor` to reject it as invalid.

This does not affect VS Code Copilot Chat or `gh copilot` CLI, which use the Responses API path where `finish_reason` is not applicable.

## Minimal repro

```typescript
import { CopilotClient, defineTool } from "@github/copilot-sdk";
import * as fs from "node:fs";
import * as fsp from "node:fs/promises";
import * as path from "node:path";
import * as os from "node:os";

async function runTest(model: string) {
const configDir = path.join(os.tmpdir(), `sdk-repro-${model}-${Date.now()}`);
await fsp.mkdir(configDir, { recursive: true });

// Resolve native binary path
const cliPath = [
path.join(__dirname, "..", "node_modules", "@github", "copilot-darwin-arm64", "copilot"),
path.join(__dirname, "..", "node_modules", "@github", "copilot-linux-x64", "copilot"),
].find(c => fs.existsSync(c));
if (!cliPath) throw new Error("No native copilot binary found");

const client = new CopilotClient({ cliPath });
await client.start();

const writeTool = defineTool("write_result", {
description: "Write a result string.",
parameters: {
type: "object" as const,
properties: { answer: { type: "string" as const, description: "The answer" } },
required: ["answer"],
},
skipPermission: true,
handler: async (args: { answer: string }) => {
console.log(` [handler] Tool called with: ${args.answer}`);
return `OK: received "${args.answer}"`;
},
});

const session = await client.createSession({
model,
configDir,
enableConfigDiscovery: false,
streaming: true,
systemMessage: {
mode: "replace" as const,
content: "You are a test assistant. Call the write_result tool with your answer.",
},
tools: [writeTool],
onPermissionRequest: () => ({ kind: "approved" as const }),
infiniteSessions: { enabled: false },
} as any);

try {
await session.sendAndWait(
{ prompt: "What is 2 + 2? Call write_result with your answer." },
60_000
);
console.log(` [${model}] SUCCESS`);
} catch (err) {
console.log(` [${model}] FAILED: ${err instanceof Error ? err.message : err}`);
}

await session.disconnect().catch(() => {});
await client.deleteSession(session.sessionId).catch(() => {});
await client.stop().catch(() => {});
}

async function main() {
for (const model of ["claude-opus-4.6", "gpt-5.4"]) {
console.log(`--- ${model} ---`);
await runTest(model);
}
}
main();
```

## Output

```
--- claude-opus-4.6 ---
[handler] Tool called with: 4
[claude-opus-4.6] FAILED: Execution failed: Error: Failed to get response from the AI model;
retried 5 times (total retry wait time: 5.36 seconds) Last error: Unknown error

--- gpt-5.4 ---
[handler] Tool called with: 4
[gpt-5.4] SUCCESS
```

Note: The tool handler runs successfully and returns a result for both models. The failure happens when the SDK sends the tool result back and waits for the model's follow-up response.

## Environment

- `@github/copilot-sdk`: 0.2.2
- `@github/copilot` (CLI binary): 1.0.27 (also reproduced on 1.0.25)
- Platform: macOS arm64
- Node.js: v22.x
- Tested models: `claude-opus-4.6`, `claude-sonnet-4`, `claude-haiku-4.5` (all fail); `gpt-5.4`, `gpt-4.1` (all succeed)

## Workaround

Catch the error after `sendAndWait` and check if the tool already wrote its output. If it did, treat the turn as successful and continue. This works because the tool call itself always completes — only the post-tool-call model response fails.

## Likely fix

Either:
1. Enable the Responses API path for SDK CLI subprocess sessions (currently feature-flagged off)
2. Fix the Copilot proxy's Chat Completions adapter to set `finish_reason` correctly for Claude tool call responses