MCP tools with $defs/$ref in inputSchema fail — parameters can't be constructed

# MCP tools with `$defs`/`$ref` in inputSchema fail — parameters can't be constructed

### Describe the bug

MCP tool calls fail with `Input validation error: 'X' is a required property` when the MCP server's tool `inputSchema` uses `$defs` + `$ref` to define parameter types. The `$ref` references are never resolved, so the client cannot construct or validate the required parameters.

Tools that don't use `$ref` (e.g., simple tools with no required params) work fine on the same MCP connection.

### Affected version

0.0.423

### Root cause analysis

The PagerDuty MCP server (`https://mcp.pagerduty.com/mcp`) returns tool schemas like this (abbreviated):

```json
{
"name": "list_schedules",
"inputSchema": {
"$defs": {
"ScheduleQuery": {
"properties": {
"query": { "type": "string", "default": null },
"team_ids": { "type": "array", "default": null },
"limit": { "type": "integer", "default": 20 }
},
"type": "object"
}
},
"properties": {
"query_model": { "$ref": "#/$defs/ScheduleQuery" }
},
"required": ["query_model"],
"type": "object"
}
}
```

This is valid JSON Schema. The `$defs` block is present in the same schema object and `$ref` points to it. However, the Copilot CLI MCP client:

1. Presents the tool parameter as just `"query_model": {"$ref": "#/$defs/ScheduleQuery"}` — the reference is never inlined
2. The model has no visibility into the actual properties of `query_model`
3. Every call fails validation because `query_model` is required but the model can't construct it

### Steps to reproduce

1. Configure the PagerDuty MCP server:
```json
{
"pagerduty": {
"url": "https://mcp.pagerduty.com/mcp",
"type": "http",
"headers": {
"Authorization": "Token token=${PAGERDUTY_API_TOKEN}"
}
}
}
```
2. Start `copilot`
3. Ask: "Who is on call?"
4. The agent attempts to call `list_schedules` or `list_oncalls`
5. Every call fails with: `MCP server 'pagerduty': Input validation error: 'query_model' is a required property`

### What works vs. what doesn't

| Tool | Has `$ref`? | Result |
|------|------------|--------|
| `get_user_data` | No (no params) | ✅ Works |
| `get_incident` | No (`incident_id` is a plain string) | ✅ Works |
| `list_schedules` | Yes (`query_model: $ref ScheduleQuery`) | ❌ Fails |
| `list_oncalls` | Yes (`query_model: $ref OncallQuery`) | ❌ Fails |
| `list_incidents` | Yes (`query_model: $ref IncidentQuery`) | ❌ Fails |

### Expected behavior

The MCP client should resolve `$ref` references within `inputSchema` (at minimum local `$defs` references) before presenting tool parameters to the model, so it can construct valid inputs.

### Additional context

- The PagerDuty server's schemas are valid JSON Schema and work correctly when called via `curl`
- This likely affects any MCP server that uses `$defs`/`$ref` in `inputSchema` (a common pattern in Python MCP servers using Pydantic models)
- Related: #1825 (another `inputSchema` handling issue)