Fixes a bug where Series.iloc assignment incorrectly converted dictionary values to Series objects when the target Series had dtype="object". This issue occurred due to improper handling in the Copy-on-Write (CoW) implementation.

[Mayankvlog]

Background

• Issue: GH#62723
• Problem: s.iloc[0] = {} would convert the dictionary to a Series, while s[0] = {} worked correctly
• Root Cause: CoW logic was not preserving dictionary objects during iloc assignment

Changes Made

• Fixed the CoW mechanism to preserve dictionary identity during iloc assignment
• Added comprehensive test suite covering edge cases and regression scenarios
• Ensured consistency between direct assignment (s[0] = {}) and iloc assignment (s.iloc[0] = {})

Tests Added

Added tests/indexing/test_iloc_dict_assignment.py with 10 comprehensive test cases:

• Basic dictionary preservation
• Complex nested dictionaries
• Multiple assignments
• Consistency with direct assignment
• Error handling for non-object dtypes
• Other object type preservation
• DataFrame iloc compatibility
• Original bug reproduction

Verification

• All tests pass
• Pre-commit checks pass (ruff, formatting, etc.)
• No breaking changes to existing functionality
• Maintains backward compatibility

Related

• Closes BUG: assignment to Series.iloc with dtype="object" converts dictionary to Series #62723
• Part of ongoing CoW robustness improvements

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New tests added

[mroeschke]

Please refer to my prior comment #62789 (comment)