docs: align Vercel S3 mount documentation with implementation

[ShawnSiao]

Summary

The Vercel sandbox implementation already supports create-time S3Mount
entries through VercelCloudBucketMountStrategy, but
docs/sandbox/clients.md still stated that no Vercel-specific mount strategy
was available.

This updates the documentation to match the released implementation.

• Mark VercelSandboxClient as supporting create-time S3Mount entries.
• Add a minimal configuration example with explicit ephemeral=True.
• Document the relevant security and lifecycle boundaries:
  • Inline credentials require explicit opt-in.
  • Sessions containing Vercel S3 mounts cannot be resumed.
  • Workspace persistence excludes remote bucket contents, and native snapshot
    hydration is unavailable while mounts are active.

Validation

• git diff --check
• Attempted uv run --no-sync mkdocs build; it timed out locally without
  output and produced no build artifacts.

[seratch]

#3872 will cover this change