fix: preserve top-level type params in FieldSchema.construct_from_dict

[mameikagou]

What

FieldSchema.construct_from_dict() currently reads type parameters only from the nested params mapping. Dictionaries that provide max_length or dim at the top level therefore lose those values, so the reconstructed VARCHAR or vector field is incomplete.

This preserves top-level max_length and dim while keeping the canonical nested params value authoritative when both forms are present. Explicit top-level None values remain ignored.

Fixes #2046

Tests

• python -m pytest -q --capture=no tests/unit/orm/test_schema.py (259 passed)
• black --check pymilvus/orm/schema.py tests/unit/orm/test_schema.py
• ruff check pymilvus/orm/schema.py tests/unit/orm/test_schema.py

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[mameikagou]

/assign @XuanYang-cn

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.22%. Comparing base (64e53d1) to head (4006b1c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3766   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files          77       77           
  Lines       16204    16207    +3     
=======================================
+ Hits        15268    15271    +3     
  Misses        936      936           

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