fix(sdk): clean up sandbox when MCP gateway startup fails - #1547

## Problem

Fixes #1498.

`Sandbox.create` allocates a remote sandbox before starting
`mcp-gateway`. If gateway startup fails, creation throws before the
sandbox object is returned. As a result, the caller has no sandbox ID to
clean up, and the orphaned sandbox continues consuming resources until
it times out.

This state transition exists in synchronous Python, asynchronous Python,
and JavaScript/TypeScript.

## Changes

- Add a rollback boundary around MCP gateway startup in all three SDK
implementations: on failure, best-effort kill the newly allocated
sandbox, then re-raise.
- Surface gateway startup failure as `SandboxError` (JS) /
`SandboxException` (Python) with a `Failed to start MCP gateway:
<stderr>` message. Previously the intended message was unreachable dead
code — foreground `commands.run` already throws on non-zero exit — so
callers got a bare `CommandExitError`/`CommandExitException`.
- In async Python, re-raise `asyncio.CancelledError` from the
best-effort `kill()` so caller cancellation (e.g. `asyncio.timeout`) is
honored; only ordinary cleanup failures are suppressed and never mask
the original error.
- Add integration coverage for synchronous Python, asynchronous Python,
and TypeScript. The tests pin the sandbox to the base template (which
has no `mcp-gateway` binary) so gateway startup genuinely fails after
allocation.
- Add a patch changeset for `e2b` and `@e2b/python-sdk`.

## Usage Behavior

No API changes. A failed creation no longer leaves a sandbox behind, and
the error is now descriptive:

```ts
try {
  const sandbox = await Sandbox.create({ mcp: { ... } })
} catch (err) {
  // err is SandboxError: "Failed to start MCP gateway: <stderr>"
  // the allocated sandbox has already been killed — no orphan is left running
}
```

## Validation

All three integration tests verified against real infra: creation
rejects with the documented error and no sandbox remains.

## Notes

Supersedes #1547 by @hxaxd (squash-merged into this branch to preserve
attribution).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: 苏紫辰 <[email protected]>
Co-authored-by: Claude Fable 5 <[email protected]>