feat(js): add experimental sandbox memory snapshot support

[willydouhard]

Summary

• Add experimental memory snapshot support to the JavaScript/TypeScript SDK
• Add SandboxSnapshot class and _experimentalSnapshot() / _experimentalFromSnapshot() methods for creating and restoring sandbox memory snapshots
• Add experimentalOptions parameter to SandboxCreateParams for server-side feature flags
• Fix missing forcePathStyle field in CloudBucketMount proto conversion

Changes

New Features

• sandbox._experimentalSnapshot(): Takes a memory snapshot of a running sandbox
• sandboxes._experimentalFromSnapshot(snapshot, params): Restores a sandbox from a snapshot
• SandboxSnapshot class: Represents a stored sandbox snapshot
• _experimentalEnableSnapshot option in SandboxCreateParams: Enables memory snapshots for a sandbox
• experimentalOptions in SandboxCreateParams: Pass experimental feature flags to the server

Bug Fixes

• Added missing forcePathStyle: false to CloudBucketMount.toProto()

Test Plan

• Added unit tests for buildSandboxCreateRequestProto with new experimental options
• Manual testing of snapshot/restore functionality with a live Modal environment

---

Note

Adds experimental sandbox memory snapshot create/restore APIs, exposes SandboxSnapshot, supports experimental options in create params, and fixes missing forcePathStyle in cloud bucket mount proto.

• Sandbox (experimental snapshots):
  • Add Sandbox#_experimentalSnapshot() to create memory snapshots and wait for completion.
  • Add SandboxService#_experimentalFromSnapshot() to restore from SandboxSnapshot with optional name override handling.
  • Introduce SandboxSnapshot class and SandboxFromSnapshotParams type.
  • Extend SandboxCreateParams with experimentalOptions and _experimentalEnableSnapshot; include experimentalOptions and enableSnapshot in create request proto.
• Exports:
  • Export SandboxSnapshot and SandboxFromSnapshotParams from index.ts.
• Cloud bucket mounts:
  • Include forcePathStyle: false in CloudBucketMount#toProto().
• Tests:
  • Add unit tests covering experimentalOptions, _experimentalEnableSnapshot, and their defaults in buildSandboxCreateRequestProto.

^{Written by Cursor Bugbot for commit f90f057. This will update automatically on new commits. Configure here.}

[cursor]

Bug: Missing validation for snapshotId from API response

The _experimentalSnapshot method uses snapResp.snapshotId without validating it exists. If the API response is missing snapshotId, it would pass an undefined/empty value to sandboxSnapshotWait and return a SandboxSnapshot with an invalid ID. The analogous snapshotFilesystem method validates resp.imageId before use with a check like if (!resp.imageId) throw new Error(...).