[FLINK-XXXXX][python] Add type hints and fix docstrings in datastream/functions.py

[Vaibhav-S-Gowda]

What is the purpose of the change

This pull request adds explicit generic type hints (IN, OUT, KEY)
to core abstract method signatures in pyflink/datastream/functions.py
and fixes several docstring issues. These methods form the foundational
public API for PyFlink DataStream users, and the absence of type
annotations was reducing IDE autocompletion quality and preventing
effective mypy static analysis on user code extending these classes.

Additionally, MapFunction, CoMapFunction and KeySelector are made
properly Generic (like AsyncFunction and WindowFunction already are),
enabling correct TypeVar binding across the class hierarchy.

Brief change log

• Made MapFunction properly Generic[IN, OUT]
• Made CoMapFunction properly Generic[IN, OUT]
• Made KeySelector properly Generic[IN, KEY]
• Added Iterator to typing imports
• Added return type None to Function.open and Function.close
• Added (IN) -> OUT to MapFunction.map
• Added (IN) -> OUT to CoMapFunction.map1 and map2
• Added (IN) -> Iterator[OUT] to FlatMapFunction.flat_map
• Added (IN) -> Iterator[OUT] to CoFlatMapFunction.flat_map1 and flat_map2
• Added (IN, IN) -> IN to ReduceFunction.reduce
• Added (IN) -> bool to FilterFunction.filter
• Added (IN) -> KEY to KeySelector.get_key
• Added (IN, Context) -> Iterator[OUT] to ProcessFunction.process_element
• Added (IN, Context) -> Iterator[OUT] to KeyedProcessFunction.process_element
• Added (IN, Context) -> Iterator[OUT] to CoProcessFunction.process_element1 and process_element2
• Fixed typo: "mthod" → "method" in FlatMapFunction.flat_map docstring
• Fixed typo: "MapFUnction" → "MapFunction" in FlatMapFunction class docstring
• Fixed doctest continuation lines: >>> → ... inside class body examples

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Verified with mypy:
mypy pyflink/datastream/functions.py --ignore-missing-imports --follow-imports=skip
Result: Success: no issues found in 1 source file

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API: yes
• The serializers: no
• The runtime per-record code paths: no
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

Was generative AI tooling used to co-author this PR?

• Yes

[flinkbot]

CI report:

• 639ab47 Azure: FAILURE

Bot commands

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• @flinkbot run azure re-run the last Azure build

[davidradl]

• is there a migration consideration with these changes?
• I am curious whether that can be tested maybe by having tests implementing the generic types.
• in terms of the PR title - the title should be either a square bracketed JIRA number or start with [hotfix].

[Vaibhav-S-Gowda]

Hi @davidradl, thanks for the feedback!

• These changes are backward compatible since Python generics aren't enforced at runtime, so existing subclasses won't be affected.
• I'll add tests that implement the generic types as you suggested.
• Will also create a JIRA ticket and fix the PR title.

Updates coming soon!

[Vaibhav-S-Gowda]

I've requested a JIRA account and will update the ticket number once it's approved.