Intermittent HTTP 500 forwarding to ManagedMcpServer (a365copilotchatmcp / Work IQ Copilot MCP) despite healthy, enabled config

### Summary
Runtime invocations of the **Work IQ Copilot MCP** managed server (`a365copilotchatmcp`, operation `mcp_m365copilot`) intermittently fail with HTTP `500`. The connector gateway reports that forwarding the MCP request to the managed server failed. In the MCP client (VS Code), this surfaces as a cancelled tool call.

The underlying MCP server config resource is **provisioned and enabled** (`provisioningState: Succeeded`, `state: Enabled`), so this is **not** a provisioning/config issue. It appears to be a transient/service-side failure in the gateway → managed-server forwarding path (`x-ms-failure-cause: Service`). The issue recurred across retries and persisted after restarting the client's MCP connection.

### Affected component
- **Resource type:** `Microsoft.Web/connectorGateways/mcpserverconfigs`
- **Kind:** `ManagedMcpServer`
- **MCP server config name:** `Work-IQ-Copilot`
- **Connector name:** `a365copilotchatmcp` (display name "Work IQ Copilot MCP")
- **Operation:** `mcp_m365copilot`
- **Auth:** `disableApiKeyAuth: true`
- **API version:** `2026-05-01-preview`
- **Region:** `westcentralus`
- **Client:** VS Code (MCP integration)
- **Gateway host:** `app-16.westcentralus.logic.azure.com` (gateway ID redacted)
- Reference: https://aka.ms/AboutWorkIQ

### Resource health at time of failure
```json
{
"kind": "ManagedMcpServer",
"properties": {
"provisioningState": "Succeeded",
"state": "Enabled",
"disableApiKeyAuth": true,
"settings": { "textOnlyContent": false }
}
}
```

### Steps to reproduce
1. Invoke the `a365copilotchatmcp` managed server (operation `mcp_m365copilot`) from a VS Code MCP client.
2. Observe HTTP `500`; the client reports the tool call as cancelled.
3. Retry multiple times and restart the MCP connection — failure persists.

### Expected behavior
The gateway forwards the request to the managed server and returns a successful MCP response.

### Actual behavior
The gateway returns HTTP `500`; the request is never successfully forwarded, despite the config being `Enabled` and `Succeeded`.

### Error details
```
HTTP 500 from connector gateway MCP endpoint
x-ms-failure-cause: Service
{
"error": {
"code": "ActionFailed",
"message": "The forwarding of MCP request to managed server 'a365copilotchatmcp' failed."
}
}
```

### Correlation / troubleshooting IDs
- `x-ms-request-id`: `westcentralus:ea07a04f-6822-4f2e-a269-6bab1709b18f`
- Timestamp (UTC): `Tue, 16 Jun 2026 23:56:41 GMT`
- Region: `westcentralus`

### Impact
Work IQ Copilot MCP tools are unusable during the failure window, blocking M365 Copilot search/retrieval workflows.

### Notes for triage
- Config resource is healthy/enabled, so investigation should focus on the **gateway → managed-server forwarding path** for `a365copilotchatmcp`, not provisioning.
- `x-ms-failure-cause: Service` indicates a service-side fault upstream of the client.
- Restarting the client MCP connection did **not** resolve it.

No repository file or test is named. Reproduce the request from the VS Code MCP client, then use the gateway host, correlation ID, timestamp, region, and failure details to investigate forwarding to the enabled a365copilotchatmcp managed server. Done means the gateway returns a successful MCP response instead of HTTP 500.

Written by the indexing model from the issue text.