[Bug]: data360 payload_examples returns UNKNOWN_EXCEPTION when toolName is omitted, contradicting its description

### Summary

On the Data 360 MCP server, `payload_examples` called with no `toolName` returns `UNKNOWN_EXCEPTION`, even though its own description says to omit the argument in order to list the available examples.

### Environment

Production org (Enterprise Edition), API v67. Client: Claude Code via a pre-registered External Client App (public client, PKCE, `mcp_api` scope). Server `data.data360`, URL `https://api.salesforce.com/platform/mcp/v1/data/data360`, activated and probed 2026-09-02.

### Detail

The tool description states, in effect, "Omit to list available examples." Calling it with no `toolName`:

```
UNKNOWN_EXCEPTION
ErrorId: 340412166-95849
```

Supplying a valid `toolName` works and returns the example payload for that operation.

### Why this matters

`data360` presents three facade tools over a very large operation catalogue (we counted 331 distinct operations across 24 families in this org). `payload_examples` with no argument is the documented way to find out what examples exist, so with that path broken there is no way to enumerate them, and a caller has to already know the operation name to get help about it. The failure is also opaque: `UNKNOWN_EXCEPTION` gives no indication that the argument is in fact required.

### Suggested resolution

Implement the no-argument listing path, or correct the tool description and return a validation error naming `toolName` as required.

Start by reproducing the no-argument call to the data360 MCP server's payload_examples tool and compare it with a call supplying a valid toolName. Trace the payload_examples entry point and its tool description; done means the documented omission path lists examples, or the tool returns a clear validation error stating that toolName is required.

Written by the indexing model from the issue text.