feat: add typed message tags

[bjarn]

Summary

• Add the exported MessageTag data class.
• Accept MessageTag objects and the existing dictionary input in sync and async builders.
• Normalize typed tags for batch requests.
• Keep tag() for the legacy single tag.

Validation

• Enforce the documented name and value formats.
• Reject reserved __lettermint names without regard to case.
• Reject duplicate case-sensitive names.
• Allow 20 total tags, including the legacy single tag.

Documentation

• Update the README, synchronous example, and asynchronous example.

Verification

• uv run pytest -q
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy src