feat: add messages resend

[bjarn]

What changed

Non-breaking. Suggested bump: minor (3.1.0 → 3.2.0).

• Regenerated lettermint.types and the operation table from the latest API spec.
• New messages.resend(message_id, body=None, *, idempotency_key=None, timeout=None) on Lettermint and AsyncLettermint. It returns a PendingSendMailResponse for the new message.
  • Without a body, the message goes to every original recipient. {"recipients": [...]} sends it to only some of them. If you pass no body, the request has no body.
  • idempotency_key= sets the Idempotency-Key header, like messages.process() and emails.send().
  • Like messages.reschedule() and messages.cancel(), it uses the team token when one is set. Otherwise it uses the sending token.
  • Errors map to the existing classes. 410 (the content is no longer retained) raises a plain APIError. RESEND_LIMIT_REACHED (429) raises RateLimitError.
• README: the resend method is in the messages table, the token note and a short example.

API changes in the regenerated types

Change	Where
New operation POST /messages/{messageId}/resend → messages.resend. Body ResendMessageRequest { recipients?: list[str] } (optional), optional Idempotency-Key header, 202 → PendingSendMailResponse. Errors: 401, 403, 404, 409, 410, 422, 429, 500	operations.py, types.ResendMessageRequest
MessageData.resent_from_message_id: optional and nullable (NotRequired[str | None]). It holds the ID of the message this one resent and is absent or None when the message is not a resend. Code that builds MessageData dicts keeps type-checking	types.MessageData
POST /send/batch items are now SendBatchMailItem, which has every SendMailRequest field plus an optional per-email idempotency_key (1-255 characters). SendBatchMailRequest = list[SendBatchMailItem]	types.SendBatchMailItem
POST /send/batch 409 can now also be an ApiErrorBody (IDEMPOTENCY_CONFLICT)	operations.py
Description updates (25 MB request limit / 413 on the send endpoints, 409 semantics)	docs only

SendMailRequest and EmailMessage are unchanged.

Usage

from lettermint import Lettermint

lettermint = Lettermint(team_token=os.environ["LETTERMINT_TEAM_TOKEN"])

resent = lettermint.messages.resend("message-id")  # to every original recipient
resent = lettermint.messages.resend(
    "message-id",
    {"recipients": ["jane@example.com"]},
    idempotency_key="resend-message-id-jane",
)
print(resent["message_id"], resent["status"])  # the new message, "pending"

new_message = lettermint.messages.retrieve(resent["message_id"])
print(new_message.get("resent_from_message_id"))  # "message-id"

Checks

• python -m ruff check src tests scripts: all checks passed
• python -m ruff format --check src tests scripts: 26 files already formatted
• python -m mypy: no issues in 33 source files
• python scripts/unasync.py --check: src/lettermint/_sync is up to date
• python scripts/generate.py --check with the generator: no differences, headers ok. Without the generator (as in CI): headers ok
• python -m pytest tests/: 246 passed on Python 3.10, 3.11, 3.12, 3.13 and 3.14
• python -m build and twine check --strict dist/*: passed. The wheel smoke test (UPGRADE.md and py.typed are in the wheel, and the client imports): passed
• Compatibility against main: every generated TypedDict has the same required keys as before (resent_from_message_id is a new optional key), and no exports were removed. A mypy --strict file with the existing send_batch call shapes passes

Not run locally

• Nothing. Every CI step ran locally.

Suggested release tag

v3.2.0 (together with the batch per-email idempotency keys PR)

🤖 Generated with Claude Code