feat: support function chains in hybrid search

[junjiejiangjjj]

• add merge operators for hybrid search
• forward function chains through sync and async clients
• allow pre-process and post-process chains alongside one L2 rerank chain
• validate merge strategy, ordering, and weighted input counts

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[codecov]

Codecov Report

❌ Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.22%. Comparing base (64e53d1) to head (ec09777).

Files with missing lines	Patch %	Lines
pymilvus/client/prepare.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3775   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files          77       77           
  Lines       16204    16236   +32     
=======================================
+ Hits        15268    15299   +31     
- Misses        936      937    +1     

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