[SPARK-54993][PYTHON] Add type hints to NameTypeHolder and IndexNameTypeHolder classes

[adith-os]

What changes were proposed in this pull request?

This PR adds proper type annotations to the NameTypeHolder and IndexNameTypeHolder classes in python/pyspark/pandas/typedef/typehints.py

Why are the changes needed?

This is a subtask of SPARK-54953 (upgrading mypy to latest version). Changes improve type checking.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Errors on typehints.py before Changes (delete # type: ignore[assignment]):

starting mypy annotations test...
annotations failed mypy checks:
python/pyspark/pandas/typedef/typehints.py:729: error: Incompatible types in assignment (expression has type "ExtensionDtype", variable has type "None")  [assignment]
python/pyspark/pandas/typedef/typehints.py:896: error: Incompatible types in assignment (expression has type "ExtensionDtype", variable has type "None")  [assignment]
python/pyspark/pandas/typedef/typehints.py:900: error: Incompatible types in assignment (expression has type "type[Any] | Any", variable has type "None")  [assignment]
python/pyspark/pandas/typedef/typehints.py:917: error: Incompatible types in assignment (expression has type "ExtensionDtype", variable has type "None")  [assignment]
python/pyspark/pandas/typedef/typehints.py:920: error: Incompatible types in assignment (expression has type "type[Any] | Any", variable has type "None")  [assignment]

After changes:
Verified with mypy that typehints.py has no type errors

Was this patch authored or co-authored using generative AI tooling?

Generated-by: Github Copilot GPT 5.2-Codex

[zhengruifeng]

@gaogaotiantian