Sub-agent MCP tool calls fail with structuredContent: expected record, received array when server returns JSON arrays

### Describe the bug

When a sub-agent launched via the `task` tool calls an MCP tool that returns a JSON array, the call fails with a Zod validation error. The exact same tool with the exact same parameters works fine when called from the primary/orchestrator agent.

The error is:
```
MCP server 'my-mcp-server': [{"expected":"record","code":"invalid_type","path":["structuredContent"],"message":"Invalid input: expected record, received array"}]
```

The root cause is in `sdk/index.js` where the `CallToolResult` schema defines `structuredContent: z.record(z.string(), z.unknown()).optional()`. `z.record()` only accepts plain objects, so when an MCP server returns a JSON array, the Zod validation rejects it. This validation only runs in the sub-agent code path, not the primary agent path.

| Context | Response type | Result |
|---------|--------------|--------|
| Primary agent | JSON array | Works |
| Primary agent | String | Works |
| Primary agent | JSON object | Works |
| Sub-agent (via `task`) | String | Works |
| Sub-agent (via `task`) | JSON object | Works |
| Sub-agent (via `task`) | **JSON array** | **Fails** |

### Affected version

Reproduced on 1.0.36 and 1.0.39 (latest as of 2026-04-29). Using @github/copilot-sdk 0.3.0.

### Steps to reproduce the behavior

1. Register a stdio MCP server with a tool that returns a JSON array:
```json
[
{ "id": "abc-123", "status": "Succeeded", "startTime": "2026-04-27T10:00:00Z" },
{ "id": "def-456", "status": "Running", "startTime": "2026-04-28T01:00:00Z" }
]
```

2. Configure a custom sub-agent with access to that tool:
```yaml
name: my-agent
tools:
- my-mcp-server/list_items
```

3. Create a session with sub-agents enabled and have the orchestrator delegate to `my-agent` via the `task` tool

4. The sub-agent calls `my-mcp-server/list_items` and gets the error above

A tool on the same server that returns a string (e.g., `"No items found"`) succeeds in the same sub-agent session, confirming the issue is specifically with array responses.

### Expected behavior

MCP tool calls returning JSON arrays should work the same way in sub-agents as they do in the primary agent. A few possible fixes:

1. Accept arrays in `structuredContent`: `z.union([z.record(z.string(), z.unknown()), z.array(z.unknown())])`
2. Normalize array responses before validation by wrapping `[...]` into `{ items: [...] }`
3. Apply the same lenient parsing that the primary agent uses to sub-agent MCP responses as well

### Additional context

- **OS**: Windows 11 (x86_64)
- **Shell**: PowerShell 7
- **Terminal**: Windows Terminal
- No workaround available from the consumer side. The `onPostToolUse` session hook fires after the Zod validation, so there is no way to intercept the response before rejection.
- Related: #2892 (MCP stdio transport for sub-agents closes after ~4 seconds, different symptom but same area)