execute/search return formatter errors for non-JSON sandbox results

## Current behavior

`execute` and `search` can return JavaScript values that cross the Worker RPC boundary successfully but are not JSON-stringifiable. The shared response formatter then throws and turns a successful sandbox program into an MCP tool error.

Confirmed real-tool examples:

```js
async () => {}
async () => 1n
async () => { const x = {}; x.self = x; return x }
```

Observed errors include:

```text
Error: Cannot read properties of undefined (reading 'length')
Error: Do not know how to serialize a BigInt
Error: Converting circular structure to JSON
```

## Reproduction / evidence

Reproduced through the real MCP tool path on the current branch state using Node 22.23.2. Each value fails through both `execute` and `search`.

The direct root cause is in `src/truncate.ts`: every non-string goes through `JSON.stringify(content, null, 2)`, then the result is treated as a string. `undefined` produces no string; `BigInt` and cyclic objects make JSON serialization throw.

Existing truncation tests cover strings, objects, large values, empty string, `null`, and arrays, but not these structured-clone values.

## Expected behavior

A sandbox program that completes successfully should produce a valid MCP text result (using a deliberate representation for values JSON cannot encode), rather than an internal response-formatting exception.

## Likely scope

This appears narrowly fixable in response formatting plus regression coverage. It should not require auth/token, deployment, dependency, or OpenAPI pipeline changes.

## Duplicate / ownership check

I searched open/closed issues and PRs for the exact errors, `truncateResponse`, undefined/no-return, `BigInt`, and circular-result behavior and did not find a matching report or active fix. Issue #68 is broader model-evaluation work and does not appear to cover this formatter defect.

If this is a useful fix, I can take it; assignment or maintainer direction would be appreciated before I implement it.