adding batch get message

[duharry0915]

Summary by cubic

Adds a batch-get endpoint to fetch message metadata for up to 500 messages in one request. This reduces API round-trips and speeds up inbox tooling.

• New Features
  • Added POST batchGet for inbox messages to retrieve up to 500 message metadata in one call.
  • Request takes message_ids (max 500); duplicates return a ValidationError. Missing or restricted IDs are omitted; compare count vs limit to detect misses. Order matches the request.
  • Response returns metadata only; body fields are not included. Use the single-message endpoint to fetch bodies.
  • Added CLI docs for agentmail inboxes:messages batch-get.

^{Written for commit fd97971. Summary will update on new commits.}

[github-actions]

🌿 Preview your docs: https://agentmail-preview-d605c116-601b-45ec-858d-1fcec7a00e1c.docs.buildwithfern.com

[github-actions]

✨ API Changes

# API Changelog n/a vs. n/a


## API Changes

### POST /v0/inboxes/{inbox_id}/messages/batch-get
-  endpoint added

💡 Download api-changelog-diff artifact or tag @fern Writer in #github-prs for changelog.

[cubic-dev-ai]

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