[Python] Fix __int__ typo and join_fn type hints in Milvus search handler

[pangwangshu]

Removes the stray MilvusBaseRanker.__int__, a typo for __init__ that made int(ranker) raise TypeError; the base class has no state to initialize, so it is removed rather than renamed. Also annotates join_fn with EmbeddableItem, which the handler actually passes in, instead of Embedding, and drops the now-unused Embedding import.

No behavior change for existing users. Existing unit tests pass.

Fixes #40474

[github-actions]

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R: @shunping for label python.

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[shunping]

Thanks for filing the bug and fix it right away! LGTM and waiting for the tests to finish before merging.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.23%. Comparing base (76f6e6d) to head (cc5ae69).
⚠️ Report is 105 commits behind head on master.

Files with missing lines	Patch %	Lines
...hon/apache_beam/ml/rag/enrichment/milvus_search.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #40475      +/-   ##
============================================
+ Coverage     56.16%   56.23%   +0.06%     
  Complexity     2288     2288              
============================================
  Files          1124     1124              
  Lines        178336   178359      +23     
  Branches       1489     1489              
============================================
+ Hits         100170   100303     +133     
+ Misses        75645    75535     -110     
  Partials       2521     2521              

Flag	Coverage Δ
python	79.69% <0.00%> (+0.13%)	⬆️

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