Preserve nanosecond precision for Timestamp

[ClausHolbechArista]

Summary

• Add NanoDatetime to preserve sub-microsecond precision for google.protobuf.Timestamp
• Route Timestamp conversion, JSON formatting, and RFC3339 parsing through NanoDatetime
• Keep generated Timestamp wrapper types as datetime.datetime
• Generate the NanoDatetime import only for known types that need it
• Typehints are still datetime.datetime to keep this non-breaking.
  • The NanoDatetime subclass still satisfies typecheckers.
  • It could be considered to add some option to set the proper type hint later.

Testing

• uv run pytest tests/test_nano_datetime.py tests/test_timestamp.py -q
• uv run pytest tests/test_known_type_imports.py -q
• uv run ruff check ...
• uv run pyright ...
• uv run poe generate
• uv run poe get-local-compiled-tests

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)