fix: convert custom Index to list in SingleData.to_series()

[Ayush10]

Summary

• SingleData.to_series() passed a qlib.utils.index_data.Index object directly to pd.Series(), which raises TypeError since pandas does not recognize it as a valid index type
• Fixed by calling .tolist() to convert to a plain list, consistent with how __repr__() already handles it on the next line
• One-line fix in qlib/utils/index_data.py

Root Cause

The Index class in qlib/utils/index_data.py is a custom high-performance index wrapper. While it supports iteration and .tolist(), pandas pd.Series() constructor does not accept it as a valid index parameter. The __repr__ method (line 618) already uses .tolist() correctly, but to_series() (line 614) did not.

Test plan

• Verified SingleData.to_series() returns a valid pd.Series with correct index
• Verified __repr__ continues to work
• Existing tests pass without modification

Fixes #2020