Set selected library to default before checks

[claudevdm]

Please add a meaningful description for your change here

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[claudevdm]

R: @shunping

[gemini-code-assist]

Summary of Changes

Hello @claudevdm, 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 refactors the pickle library selection mechanism within Apache Beam's Python SDK. By moving the resolution of the 'default' library option to an earlier stage in the set_library function, it improves the robustness and predictability of the library selection process. This change ensures consistent behavior when specifying the default pickler, particularly before other library-specific checks are performed, preventing 'default' from being treated as a literal library name in subsequent logic.

Highlights

• Pickle Library Defaulting Logic: The logic for resolving the 'default' pickle library option has been moved to the very beginning of the set_library function in pickler.py. This ensures that if 'default' is specified, it is immediately translated to DEFAULT_PICKLE_LIB before any other library-specific checks are performed.
• Code Refactoring: A redundant check for the 'default' pickle library was removed from later in the set_library function, streamlining the code and preventing potential inconsistencies.

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[shunping]

LGTM. Will merge once the tests are green.