Fix 37351 dofn cache collision

[Eliaaazzz]

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Summary of Changes

Hello @Eliaaazzz, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical cache collision issue within the Apache Beam Java SDK's ByteBuddyDoFnInvokerFactory. Previously, the system could incorrectly reuse DoFnInvoker instances for DoFn classes that shared the same base class but differed in their generic type parameters, leading to potential runtime instability. The core of the solution involves refining the caching mechanism to consider the full generic type information of a DoFn when generating and retrieving its invoker, ensuring type safety and correct behavior across diverse DoFn instantiations. A minor change was also made to the Python SDK to improve type hint handling.

Highlights

• Java SDK DoFn Invoker Cache Fix: The ByteBuddyDoFnInvokerFactory in the Java SDK has been updated to prevent cache collisions for DoFnInvoker constructors. Previously, DoFns with the same class but different generic type parameters could incorrectly share the same cached invoker.
• Enhanced Cache Key: A new InvokerCacheKey class has been introduced. This key now incorporates the DoFn class, its input TypeDescriptor, and its output TypeDescriptor, ensuring that different generic instantiations of the same DoFn class are treated as distinct entries in the cache.
• Unique Generated Class Names: Generated DoFnInvoker classes now include a hash of their generic type parameters in their names. This prevents class name collisions at runtime when the same DoFn is used with varying generic types.
• New Test Case for Collision Proofing: A dedicated test (testCacheKeyCollisionProof) has been added to verify that different generic instantiations of a DoFn correctly produce distinct invoker classes, confirming the effectiveness of the fix.
• Python Type Hint Refactoring: A minor refactoring in the Python SDK's external.py to store type hints in a local variable, preventing potential AttributeError and duplicate calls during processing.
• Gitignore Update: The .gitignore file has been updated to include .venv/, preventing local Python virtual environment directories from being committed.

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