fix: prevent payload mutation and parallelize batch rendering

[Ray0907]

• Fix payload mutation in emails.create(), templates.create(), and
  batch.create() — these methods currently modify the caller's input object
  (payload.html = ..., email.react = undefined), which can cause bugs when reusing the
  same options object across multiple calls
• Parallelize batch rendering — replace sequential for...of + await with
  Promise.all, so React template rendering for batch emails runs concurrently instead of
  one-by-one

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Summary by cubic

Prevents payload mutation across email/template/batch APIs and runs batch React rendering in parallel to speed up sends. Removes side effects when reusing options objects and lowers batch latency.

• Bug Fixes

  • Stop mutating caller input (payload.html, email.react) in emails.create(), templates.create(), and batch.create(); use immutable copies.

• Performance

  • Parallelize batch React rendering in batch.create() with Promise.all instead of sequential awaits.

^{Written for commit 24647f5. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[felipefreitag]

Thank you, superseded by #862