[fix][fn] Align WindowContext with Context

[jiangpengcheng]

Fixes #23629

Main Issue: #xyz

PIP: #xyz

Motivation

Modifications

Verifying this change

• Make sure that the change passes the CI checks.

• This change is already covered by existing tests, such as (please describe tests).

  • org.apache.pulsar.tests.integration.functions.java.testTumblingCountWindowTest
  • org.apache.pulsar.tests.integration.functions.java.testSlidingCountWindowTest

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

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
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• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: jiangpengcheng#35

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