The `#[serde(default)]` on `CallToolResult.content` (added in #752) made
all fields optional, causing `CallToolResult` to greedily match any JSON
object during `#[serde(untagged)]` deserialization of `ServerResult`.
Similarly, `GetTaskPayloadResult(Value)` matched everything before
`CustomResult(Value)` could be reached.
Fix by replacing derived `Deserialize` impls with custom ones:
- `CallToolResult`: require at least one known field to be present
- `GetTaskPayloadResult`: always fail (indistinguishable from
`CustomResult` in JSON; construct programmatically via `::new()`)
