feat: add per-email idempotency keys to batch send

[bjarn]

Stacked on #36 — merge that first.

What changed

Non-breaking. Suggested bump: minor (3.1.0 → 3.2.0, released with #36).

• New lettermint.BatchEmailMessage type. It is an EmailMessage with an optional per-email idempotency_key, and it mirrors the generated SendBatchMailItem.
• emails.send_batch() now accepts Sequence[EmailMessage | BatchEmailMessage | EmailBuilder], so lists typed as EmailMessage still type-check. A per-email key goes out as the item's idempotency_key field.
• The idempotency_key= argument of send_batch() is still the Idempotency-Key header of the whole request. emails.send() and builder.send() never put idempotency_key in the body.
• To give a builder a key, spread its wire format: {**builder.build(), "idempotency_key": key}. BatchEmailMessage.attachments also accepts the base64 attachment list that build() returns, so this type-checks.
• The SDK adds no new client-side checks. The API checks that keys are 1-255 characters and unique within a batch. Reusing a key with different content raises ConflictError with code IDEMPOTENCY_CONFLICT.
• Docs: the README batch section and the send_batch docstring cover per-email keys. They also explain that on a partial failure, the error's details["accepted"] lists the emails already accepted (index, message_id). The existing APIError.details already holds that list, so there is no new accessor.

Usage

from lettermint import BatchEmailMessage

john: BatchEmailMessage = {**welcome.to("john@example.com").build(), "idempotency_key": "welcome-john"}
results = lettermint.emails.send_batch(
    [
        {"from": "hello@acme.com", "to": ["jane@example.com"], "subject": "Hi Jane", "text": "Hello",
         "idempotency_key": "welcome-jane"},
        john,
    ],
    idempotency_key="newsletter-2026-10",  # still the Idempotency-Key header of the request
)

Checks

• python -m ruff check src tests scripts: all checks passed
• python -m ruff format --check src tests scripts: all files formatted
• python -m mypy: no issues in 33 source files
• python scripts/unasync.py --check: up to date
• python scripts/generate.py --check with and without the generator: ok
• python -m pytest tests/: 250 passed on Python 3.10, 3.11, 3.12, 3.13 and 3.14
• python -m build, twine check --strict dist/* and the wheel smoke test: passed
• Separate mypy --strict runs: existing calls type-check unchanged (list[EmailMessage], list[EmailMessage | EmailBuilder], tuples, async builders), and so does the README snippet

Not run locally

• Nothing. Every CI step ran locally.

Suggested release tag

v3.2.0 (together with #36)

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