[ty] Avoid MRO cycle when collecting NamedTuple fields (#26464)
## Summary
When we build the MRO for a class-syntax `NamedTuple`, we synthesize its
tuple base from the class's own field types. `own_fields` previously
queried `typed_dict_params` unconditionally, even though only
`TypedDict` field generation uses those parameters. That query checks
whether the class is a `TypedDict` by walking its MRO, re-entering the
MRO construction already in progress.
This evaluates policy-specific field metadata only for the field kind
that consumes it: the `TypedDict` `total` default is read only for
`TypedDict` fields, binding defaults are inferred only for `NamedTuple`
and dataclass fields, and `KW_ONLY` sentinels are checked only for
dataclasses. In particular, gating the `typed_dict_params` lookup
removes the `MRO -> own_fields -> MRO` dependency without changing field
generation. The recursive generic `NamedTuple` from the issue is covered
under Python 3.14.
The full ty semantic test suite, package Clippy, and repository hooks
pass.
Closes https://github.com/astral-sh/ty/issues/3872.
