Some minor type fixes/clean ups

[NoodlesNZ]

Applies the valid type fixes from #313, dropping changes that were superseded by the Pydantic refactor in #320.

Applied:

• flows.py: Fix req_reply return type annotation ((DF, DF) → tuple[DF, DF]) and reply return type (DF → tuple[DF, DF])
• template_engine.py: Rename format_field parameter spec → format_spec to match string.Formatter base class signature
• pytm.py: Import PydanticUndefined from pydantic_core instead of the private pydantic.fields module
• tm.py: Declare _duplicate_ignored_attrs as a ClassVar rather than monkey-patching it at module level

Dropped (no longer applicable):

• varBool string-to-bool coercion — varBool is now a plain type alias; Pydantic handles coercion
• DataSet.str None guard — DataSet was rewritten in Refactor models to Pydantic #320 with a _names index that is already safe
• DataSet generic type parameterization — superseded by the new DataSet implementation in base.py

Tests
All tests passing, no additional code coverage needed.

[izar]

LGTM!